I can't see anything with that picture. Please post a more clear picture or explain what your seeing. Also give us temp, chemistry, and recent changes or additions to the aquarium
Looks like it's infected or something. Before you medicate dose the tank with aquarium salt (if that's your only fish) or dose a quarantine tank. If that doesn't work maybe medicate with melafix or pimafix but it's always best to try salt first.