is it white cottony stuff??? if so, then it's most likely a fungus infection. It is advisable to treat it immediately since the infection is near it's mouth. That will deter the fish from eating and it may die of starvation.

As for the right kind of medication, you can consult your LFS on that. Just remember, do not simply mix the medication, it can turn out to be a toxic mixture for your fish.

Good Luck!