actually, mine has it as well.
i was reading about it.
im pretty sure your fish has an internal parasite.
you have to treat the water in order to help your fish get healthy again.
im not really understanding what its from. but it seems to be something with
rotting plants or rotting food ( im guessing when the food is left at the bottom of the tank for too long and then the fish eats it ?? )
i dont know.