pest species, aiptasia, etc: some have problems; some don't: why?

Everybody has certain unwanted species arrive. NOT everybody has them take off and become a pita.
An exception is caulerpa algae in your rockwork: THAT is a plague that can cost you all your rockwork if your tank is too small to accommodate one of the only species that will eat it (rabbit).

Hair algae is a piffle. Most everybody gets it. A tank that continues to have it past a few weeks has a lot of phosphate it shouldn't have. Instead of buying a critter alleged to eat it---get a gfo reactor. Doesn't work for you? There's a secret: change the media!!!!! In a bad case, it gets full-up in about a week. If you at least change the media out once monthly until you see bare rocks, you'll get good results from your reactor: tune it until just the surface of the medium moves just slightly.

Other pests are a problem ONLY if you intend to keep species they eat. Examples, nudies that bother zoas don't bother other corals. Asterinas that may nosh on zoas don't bother a thing otherwise. And conversely---thinkaboutit---without those prey species, they don't--can't--- multiply like crazy either. Asterinas eat your coralline? You should generally be so lucky. Coralline can get all over your glass as well as your rock---a pest in itself. But nice on the rock.

Other general pests that feed off excessive nutrient in your tank---can be controlled by not having excessive nutrient in your tank. Yea, verily---get a better skimmer, lower your amino acid level, and lower your nitrate into the teens or ones (if you have stony coral or fish) fives (if you have lps or fish)or 20's (if you have zoas/softies or fish), and you generally won't have too much problem with these either, while your corals will more easily color up and take off. Pests that are in this class of nitrate-loving are aiptasia, majanos, and, I strongly suspect, cyano and bubble algae.

That leaves you a very few sincerely bad-news items like flatworms (Flatworm Exit or certain wrasses) and red bug (an acropora headache) that require heroic measures to get.

But in general pests have to eat to reproduce. If your tank isn't feeding them, they won't become a problem. It may be the species you're keeping: in that case, be careful. It may be your nitrate level: have enough live rock, use NoPox, and get a really good skimmer; it may be you're feeding too much---ask your lfs, for one. But just say to yourself: 'somehow I'm feeding this. What is it eating? and How is my tank providing it?'