It could be just some non shed skin, if it's just a patch of dead skin
that didn't come off it will get fuzzy with fungus (yes, not chytrid)
Yes, you can try to wipe it off with a Qtip.

As for the 3 gallon, are you doing 100% changes?
I am always concerned about using filters on 3G or smaller tanks
because they are very difficult to keep a cycle going.
They always seem to short circuit,
leaving residual ammonia and nitrite.
Have you checked levels lately?