looks like a sponge actually, if it's fixing nitrogen (bubbles) its bacterial
Tough for me to tell from the pic right now (been drinkin he he) but if it's got that pale yellow/tan sorta look and kinda slimy squishy to touch it's probably the same sponge I've seen in a few tanks now. You can cut it out pretty easily but it will continue to grow. Might have a look at
Sponges
