looks like a little spot of ick to me. I'ld treat your whole tank with maracide, quick cure (malacite green and formalin), or just straight malachite green depending on what species you have in the tank. Ick drops of the fish and lives in the gravel for a part of it's life cycle, I would personally treat the whole tank. Raising the temp also speeds up the life cycle and makes treatment easier. If it is ick it is fairly easy to treat, good luck 
