Silver Shark 4" Growing White Fluffy On His Body

[randal2k12]

hi my name is lee, i was wondering if someone could help me as my silver shark about 4" has all white fluffy stuff on his side, i cleaned it yesterday and its grown back today, i brought it of my girlfriends mum and she brought it from a pet shop but she only had it for one day becasue the fish was too big for her tank so i brought it but the shark has some bight marks and a few scars on him thanks for any help.