Then it must be some type of fungus.. I would start looking up for fungus treatments .. I had an angel fish that had white spot on its anal fin it acts normal and ate normal.. I treated for ich but it didn't get better nor any improvement .. Then I started treating for fungus and I noticed some improvement.. However I couldn't get rid of it.. I tried for over a month and switch meds three times before I finally had to put it down.. It's tough to treat fungus.. So I would start ASAP .. Check ur other fish .. Check tails, fins , and mouth..