Thanks for the response. Definitely not algae. This has been around for years even after an acid wash, bleach wash and power wash. Just recently I used a 3500 psi pressure washer and no change. Will not scratch off either.
They may have been in the original plaster and creamed and troweled over. Now that the plaster has gotten older and the water has slowly eaten away the cream you see these. Its on the edge of the steps but not elsewhere in the pool?