Okay, Ned's idea set off a bell in my head... i have seen a very similar structure in the zebrafish facility where I used to work.
I would look under a microscope or at least a magnifying glass and see if it looks like there are "polyps" that the animal retracts and then later expands again. If you are familiar with the smell of coral, you will find that when you crush the growth it smells similarly.
I think Dan is right.... only, it's a cnidarian, like a coral. I bet its either a hydra or a Bryozoa. They grow in fresh, marine, and brackish water and thrive in tanks with lots of particluate food. Baby brine shrimp are exactly the right size and that's what they lived on in our zebrafish nusery. In your tank, it could just be a little over feeding. Eradication is tricky with live plants, snails, and sensitive fish. But do a little research and look into plant dips if the problem continues.
I cant find a good picture, but heres some similar creatures:
http://depts.washington.edu/fhl/assets/ ... yozoan.jpg
http://members.optushome.com.au/chelmon/Hydra.htm
http://www.thekrib.com/Fish/hydra.html
Hope that helps,
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