It looks like mustard algae. On some pools, it seems to take root almost instantly, if you let chlorine levels drop. Once present, it doesn't die unless levels are VERY high.

It's difficult to eliminate, but fairly easy to control on vinyl pools -- moderately high chlorine plus careful brushing will eliminate 99% of it quickly. BUT . . . it's not gone, and will return at the first opportunity.

It only lives where nothing disturbs the surface it's on -- thus, it often begins on the side walls in the deep end.