those snails are called nassarius snails they eat dead and decaying tissue you need to remove him right away and QT he does have some kind of fugues or perisite from what you are saying the nassarius snails can eat him alive if he can not get away from them when they approch. nassarius snails can be pretty fast for a snail. He could be on his way out and you do not want to infect the other fish. what type of fish is the other one? Can you snap a pic and post it there are so many things that can be doing this to him including a fish bit and now the nassarius snails know they can get a meal off him they may not leave him be.

Bill