Can I assume two things from your user name and signature that you are in the inland desert area of CA, and the pool is only 3 years old? The reason I ask about this assumption is the potential high cost to you for another drain, repair and refill.
It looks to me to be a case of a few small cracks in your gunite allowing water to reach the rebar in that specific location in your pool. If you were inclined and able to poke into that expanded area (I really wouldn't with the water in the pool!!!) if it were dry, you would find the same type of rusty scale that the plasterer removed above it.
Is there a way you could take a picture of it?
Another thing short of poking it would be to take something solid and VERY GENTLY tap around the edges of the expanded area and listen for a change in sound from the edge of the area to a known solid part of the pool. This would confirm that you have at least a delamination of the plaster from the gunite. If there is a sound change, it would be time to call the plaster guy back to fix before it flakes off.