Welcome back. There are some really nice looking saltwater fish, but they can't even come close to the personality of a freshwater cichlid.
This is what I do as well. I throw in 3-5 pinches of NLS Large fish formula, and they'll grab a few whole pellets themselves, then munch on what my JD/FM/Con spit out. They'll also nibble on the zucchini that I drop in once a week for my bristlenose.
As for what could be doing that, I have no idea. How long have you had them? Almost looks like a fungus or something. One of my largest silver dollars got a nasty bump on him when I first put him in the tank (he got spooked and ran into a piece of driftwood) but since I've increased the school to 5, he's calmed down a lot.
