It's hard to say what it is. It could be physical damage, bacteria, or fungal infection. I recommend you let the sailfin be for a couple of days and see if the problem worsens. If it does, post another pic of the diseased area so we can better diagnose what the problem is.
I've had experiences with two fish before that had two unique patches on their sides and I assume they were just scratches or from fighting. They both cleared up after about a month. I agree with Len, if it gets better on it's on or just seems to stay the same, let it be. If it gets worse, take action.
I just get a sailfin tang i don't know what this jel like stuff is, if i did this right i sent a pic Please help!
