Dio,
If it's slimy or just sort of like a scum it's probably algae. Although I
shouldn't think a small amount would be any problem, if it becomes really
widespread it could start up some decay problems as the lower portions of
the algal layer start to die off. Perhaps a bit of aquarium algicide in
the water would slow it down. I don't think this will affect your VFT
since it's safe for aquarium plants - anyone ever try this? On the other
hand, if it appears to have very tiny leaflike structures, it may be
another species of moss. In that case, you'll just have to remove it by
hand if it starts to take.
Liane Cochran-Stafira