I think you scrubed it enough. Leave it in there and let the cycle take it to it's end. It's some type of fungus and when the food supply goes down it will disappear. As far as water changes, the only reason to do a PWC
is if you saw something on the LR
that you wanted to save or you used straight ammonia and overdid it. Very few good bacteria live in the water column. It's on the hard surfaces that the bacteria grow.
PS: On a second look at your photos, it could be some form of sponge too.