It doesn't seem to retract when touched and it's not green, it's more translucent/white. I've left the rear glass to see what it develops into and it doesn't get bigger than 1-1.5mm and they are spread a few mm apart and look lke inside out umbrellas without the fabric! In between there is a thin film of translucent stuff. I'd take a pic but it's so small and my camera is so basic... The translucent film I can imagine is something to do with snail tracks and maybe something is growing on that? It's a 240l tank and only three apple snails (anyone want 3 apple snails?!) so I can't see how they've covered the whole tank!!