Algae above water line?

[cschnurr]

Last weekend, I brushed the coping and walls above the waterline.

In a couple spots I noticed some miniscule yellowish brown deposits on the coping. When I brushed them off, they dissolved fairly easily and the colour was very intense like I would expect from a dye.

Is this mustard algae?

[Simmons99]

algae will have a slimy feel - did it come back?