Dusted off the test kit. Now I remember the pH thing that always struck me as weird and made me nervous about large water changes. Tank pH 6.0 (or less), tap water 7.8. Nitrite 0, nitrate 5-10, KH 1 which doesn't seem right but it's not the freshest test.
I don't know if those numbers are conducive to the green spot algae but I guess it's time to scrape off the green stuff (which apparently requires a taping knife and not a mere spatula and that is not a good thing when the stuff is next to sand scritch scratch) and do a little water change. And order some bulbs. And fishy sunglasses. Poor guys have been in the dark so long...
So aside from a little maintenance, is there anything in particular that would perhaps possibly slow the advance of the green spot?