How big is your tank? How big is the ID shark? You do know that they get to at least 2 feet long? They are really jumpy and delicate fish and really should not be kept by inexperienced fish keepers. I know because I made the same mistake myself. I have no idea what could be ailing him. It does sound a little like ich, in which case you are going to have to treat.
ID sharks need a fast current to be happy and a really big tank, we're talking at least 250G, maybe more. I am sure that there are some that will disagree with that, but I have seen a bigger one at the lfs and they are huge. The ones I saw were about 18" long and at least 6" wide. They don't stay silver when they get big, they turn black. And they stop looking so cute too. They need lots of room to be happy and lots of hidey holes to feel safe.
Can you post a picture of the shark? Sorry I couldn't be of more help.