My gunite pool has pits in the plaster averaging 1" in dia. now in these pits I find what appears to be red algae. Have never seen this before. The water is sparkling clear. Regular additions of algacide and a correct chlorine level don't seem to help. shock doesn't help. the red can be scrubbed off with a wire brush, but keeps coming back. Any ideas?

I'm new here, have lurked for a year or two, but this is really the first problem I've had with the pool since I moved in 3 years ago.
thanks in advance,

Chris