Does it wipe off easily? I've got a 10g with lots of tiny snails in it. I find I get lots of brown fluffy mulm that settles out on everything without a strong current across it.

It seems like it shows up overnight (when snails can be active) so I'm pretty sure it's due to the snails because I don't see it in any other tanks.

Doesn't really bother me much unless I fall behind in my water changes.

If it wipes off realtively easy but feels a little greasy then diatoms are a logical assumption.

For diatoms I've found that a few ottos will take care of it quickly. It seems to be their favorite food.