The BBA is a fairly minor issue in a tank like this: just on some old growth that needed trimming away anyway. If need be I'll use some excel, but the problem should be more or less resolved.
I've found that BBA tends to show up initially when fish are added and co2 levels are modified to adapt to that. Swift removal then tends to make sure it doesn't come back.
Another correlation is BBA showing up at about the time the tank's growth is full and stabilized, again a sign of needing to modify dosing and co2 regime.
But to answer the question it was hanging out a little on the big rock.