I've noticed that a green film or color is forming on rocks at the bottom, objects, decorations, and slightly on the glass. Earlier today I did a 50% water change and a filter change. It was hours after this I noticed it's forming more prodminently on various things in my tank. A)What is it? B)Is it harmful? C)What can I do about it? D)Will getting a pleco help? (I had one and it died a little while back. This never happened when he was around) Thanks, much help appreciated!