hard to tell, does it sway and move with the current ?It doesn't look lie planaria t me... I had something similar on my 55 when first set up and stocked... Just kept wiping it off for a couple of weeks with a cloth/razor/scrub brush what ever was handy at the time of the water change.
So it's just white algea buildup? Never had it before just wondering. I have been doing weekly 30% water changes on my 180 which is about 50-60 gallons
well this will make you worry, I have some pics of mine that I know is white algae cuz there is a bit on the wood. Also some on my macaw tho so idn, lol.