Also pollen can look yellow, especially if it combines with cottonwood or ragweed "cotton" in the skimmer. They you get a yellow "mat" (for what of a better word) in the skimmer basket. BUT, it doesn't feel slimy, it actually almost feels like shredded cloth. Definitely not mustard algae.

We are also still landscaping around the pool, and the very fine silt that constantly blows in will cover the walls as well as the floor. It looks somewhat yellowish when it is in the pool.