Looks like a mouth fungal infection (actually a bacteria). What type of substrate are you using? They are burrowers, and even with gravel they will do their best to dig in. Gravel really is not good for them as they injure themselves head butting it.
Its color looks a bit washed out. Is it just the photo or is this the way it really looks?
How many do you have in the aquarium? They are very social fish and need to be in a group. When the numbers are two small they can get frantic trying to find a place to hide.
What are you feeding it?
What are your water readings?
Do not use salt. They are scaleless and salt is bad for scaleless fish.
EDIT: I just checked your profile. You have four types of fish that need to be in schools of around six or more. If they are in proper sized schools you are over stocked for a ten gallon aquarium. If this is the case, and the betta is a male watch for aggression from the betta.