Posted: 06/10/2012 at 09:47
I have the same stuff on some mossy staddle stones and yesterday discovered it all across MIL's patio. Her algae has some very large pieces and also some small blackish bubbles. It is lethal to walk on and has covered her patio very quickly.
Just as a thought - two weeks ago we sprayed her patio with Roundup to get rid of grasses and other weeds in the cracks. I suppose Roundup hasn't got something in it which has caused this?