sick black Moore

i have a black Moore that is getting a white spot on him its rely small but it looks almost like a fiber fungus type thing. now, this has happened two times before this the first time i just left it and it went away the second time i had a powdered medicine i put in the tank and eventually it went away should i just let it go like the first time has he built an immunity to it i rely don't know if i need to i will try and put pics

 

Were you able to get any pics and how is he doing?