Please help. What's happening to my flagfin?

I've had a flagfin angel for about 2-1/2 weeks. He appeared healthy and was eating well at the LFS, and has been healthy and eating well since I got him. Just a great fish.

Yesterday I noticed a small white blemish on his left side. Today I came home and it has grown significantly. It appears like something is eating him away. See the pic.

Any idea what this is and how to treat it? I'll be devastated if I lose him, and certainly dont want any other fish to get it either.