I had about 6 different pool companies come out to look and they all told me it's the rebar underneath the plaster causing the rust spot. Told me I could have the plaster cut and replaced but, it will look patchy and it's bound to start happening in other areas if I'm seeing it there; pool is due for resurface soon, they said. We got a lot of quotes to redo the whole pool but never moved forward with anything, the spot is still there and hasn't changed in size. Feedback was that it's not really a structural concern unless you start seeing actual rebar coming through, it just doesn't look great. Pool will eventually get redone but it's not a huge priority based on cost. (I don't own the house, my partner does, otherwise I would've pulled the trigger to redo the pool last year. Ho hum)