Pink tint on plaster pool steps that won't scrub off

Has anyone had a pink color appear on white plaster pool that won't come off? I have put vitamin C tablets on it and when the vitamin C is picked up, there is a white circle remaining where the tablet was placed. I thought that meant there was something organic in the pool. It won't scrub off and it is not slimy. What is it and how do I get rid of it?

I am wrapping up a "Slamming" session and the pool looks good otherwise.

17,000 plaster pool
FC 3.4
CC .5
pH 7.4
TA 100
CH 250
CYA 55