Does mustard algae take different forms?

About 5 days ago I took off the solar cover to find a bright yellow patch about 3 feet long where the side and bottom meet in an area that doesn't get much circulation in the deep end. Following all the good advice on this board, I started to shock and brush, still am. The yellow is gone, but I'm getting little quarter-size patches all over the bottom of the pool in the depressions (vinyl over sand, never did get all those foot-prints out). These patches are brownish-gray and I brush, but they come back in a few hours. Is this still live algae or is it dead? Mind you, my pool has remained clear, never turned cloudy during this time. If it's alive, and I'm guessing it is, since I'm losing about 3 ppm of chlorine through the night, why was it yellow and now it's gray?