the green stuff is algae and is extremly difficult to remove
I too let mine go, but after time I hated it and your right, it dont budge. If one day you do decide to try and get it cleaned of here's what I did earlier today. I got one of those flat scrapers with the blade and removed what was visible from the looking onto the tank and left what was behind the rocks for my angel. Looks sooo much brighter, I am very pleased. Oh yeah, either have a net ready to catch it or be ready to syphon and in turn a water change. Just sharing what worked well and fairly fast for me....