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.
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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.
 
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