It could be a number of things, and would be best if you could most some more info. Here's some thing it could be:
1) If it looks like tiny bubbles, then it's an over-abundance of dissolved oxygen. Nothing too major as it will sort itself out. It can get to be a major problem though if it does. Over saturation could be from over filtration, lack of surface area, or saturation of a chemical (perhaps ammonia) that isn't allowing the gas exchange on the surface. I doubt this is it since you would have noticed bubbles instead of spots.
2) Stage of ammonia toxification, though it's normally red streaks or black spots. Red streaks can also be from interal parasites/bacteria. The black spots mean that the fish was burned by the ammonia, but it healing now. White spots may also be another (less seen?) symptom.
3) Fungus. Most "fish" fungi will produce a white (sometime fuzzy) appearance.
4) Fin rot. I doubt it's this since you made no mention of the fin actually rotting.
5) Ich. Quite possible.
6) Parasite. I also doubt this since a white parasite is usually seen dangling/trailing behind the fish and not in a "spot".
7) Bacteria. I'm not sure on different strains, but it's possible.
If you can give us a better description of it, we can narrow it down for you. Right now, based on my knowledge, it sounds like a Fungal infection.
The ammonia is probably dropping the immune system enough that everything will attack/latch on. Keep your ammonia lower than 1 ppm
(actually, .50 ppm
would be better). Do a PWC
as needed to help, but try to stay away from things like AmmoLock if at all possible.