I posted something similar to your situation, but haven't received a response: http://forum.simplydiscus.com/showth...rst-week-Video
However, from browsing around, I believe it's fungus. Here's an older post that talks about it: http://forum.simplydiscus.com/showth...te-spot-on-fin
I had white fungus growing on my new driftwood, although I read that it wasn't harmful to our fish. Do you have any wood in your tank?