I fished with a fisheries biologist that said it was called Lymphocystis. He told me the slime on the crappie usually protected the fish from this, but if it gets rubbed off for some reason ,they are more susceptible to getting it.
It's not red spot disease, so I can't tell you exactly what it is, looks like a fungus, I've caught a few with that on them myself, never bothered me when I'm eating them, frying should kill anything bad.
Supposedly it's a fungus that occurs from stress...changing water levels...temperature...spawn...whatever stresses a fish. It seldom reaches down into the meat but if it does just cut it out. The fish is ok to eat. We do see it often in East Texas...some lakes more others.