Spots on fish?

We have a 10 gallon tank with mollys' in it. The tank is cleaned every weekend with a major water change at least once a month. Some of the fish have been developins over the last month or so. The lighter colored fish have white blotches on their fins, and the darker colored fish have black spots on their body and fins. None of these spots resemble salt, so I don't think it's ich/ick. The only way I can really describe what the white spots look like is spilled milk on the fish. Any ideas what this could be?