My Occ. clowns are starting to develop black blotches on the mouth area and sides of head going down the side of the fish. The biggest area is around the mouth. I was wondering what this could be caused by?

Everything else looks good. The frogspawn, zoos, and green stars are growing like crazy and look better than ever.

I feed a mixture of pellets and frozen brine shrimp and also use chromaplex everyonce ina while for the corals?

Oh the tank is a 12 gallon JBJ nano cube, I'm not sure what the problem is, any ideas?