Before I begin, here's a link to a listing of Stirner's uses of the term "spook". You don't need to follow each citation, but at least review the two lists.
Now that you've read that, I want to expand upon the idea. As I try to indicate in the document linked to above, a spook, at least as Stirner uses it, and as I think the term is most usefully meant, are ideals.
I also need to bring up the fixed idea. Unlike the term "spook", which is explained mainly through repeat examples, Stirner is pretty clear about what he means by a fixed idea:
What is it, then, that is called a "fixed idea"? An idea that has subjected the man to itself. --The Ego and Its Own
Both spooks and fixed ideas are spoken of as if they possess a person, as a spirit or demon might be said to possess a person. In the case of religious examples, this meaning lends itself to a bit of confusion, the analogy seems more natural in a way that specific nuance can be mistaken for being part of the religious frame of the wording.
An ideology is commonly defined as "a system of ideas and ideals". Those vaguely familiar with moral philosophy will recognize the distinction being made. For those less familiar reading this, it's often called the "is-ought" or "fact-value" distinction, the difference between a description and a prescription, between a position and a norm, between what actually exists today and what one imagines would be better. In a relevant way, what we call religion isn't mere spirituality, what we call religion is spiritual ideology. We don't call those who believe in ghosts but who worship no gods "religious", we may call them "superstitious", but we reserve the term "religion" for those who feel compelled to follow the ideals that their gods represent. This feeling of compulsion to follow ideals is exactly what Stirner was speaking about when he discusses the possession by spooks and fixed ideas. I believe that by both the terms "spook" and "fixed idea", Stirner was aiming at what we today call ideology. The word "ideology" does not appear, at least in the Byington translation, to let us distinguish or compare the two.
So in a way we can describe both of them as "ideology", which I believe improves the terminology quite a bit by replacing special jargon words, but also expands the term "ideology" to include things that we don't often recognize immediately as ideology, but which still are. Gender roles are just as much ideology as Christianity is.
But if we can describe any system of ideas and ideals as ideology, what does that make our mind? As we think, we keep in mind both what we observe and can describe, and what we find to be the ideal we want to move towards. Does that make consciousness ideology? In one sense, yes, although we more often mean large contiguous systems of ideology when we speak of it, such as liberalism or communism, that's still just a connotation. If we only call the mass-appeal spooks and fixed ideas ideology, we're left without a term sufficiently generic to describe even the smaller examples, and I think there's value in grappling with both large and small ideologies at the same time using the same term, after all, they have enough in common.
So is ideology bad? What would it mean for ideology to even be bad? Wouldn't ideology being "bad" require another system of ideals and framework for analysis of ideas to be presupposed, and thus, assume an ideology in designating ideology as bad? It would. And perhaps there are Stirnerite egoists out there who think that ideology is in some abstract way "bad", but I haven't met them. So why should we care about ideology?
It can only be for your own reasons. Do you want to be a servant to an ideology? Are you satisfied that you are free if no other person is your master, even if you find yourself serving the purposes of an ideology in ways that limit your satisfaction of other goals? Are you held back in your expressions of compassion by your desire to appear masculine? Are you reserved about enjoying a sport that looks like fun because it's not very feminine? Are you worried about whether your attraction to somebody makes you not straight, or not gay? Do you fear that your temptations and thoughts might lead you to sin? Do you find yourself stopping your own thought before you take a political stance that would be unpopular because it wouldn't fit with what everyone else thinks a "real leftist" or "real libertarian" or "real republican" or "real communist" or "real democrat" or "real American" or "real Texan", would say? If you answered yes to any of these questions, you are limiting yourself in service to the moral imperatives of ideologies, abstractions which you give power to while thinking yourself free.
So why should you care about ideology? Because once you recognize ideology for what it is, ideology becomes your slave, not your master. It is your tool to wield as you shape the world to your liking, you are not its instrument of self-realization. And no matter what your specific values are, you now have greater power to achieve them, you have overcome the limits you imposed upon yourself.