Good advice from topless. As for how to distinguish between green algae and yellow/mustard algae, it's usually easy because green algae is usually free-floating and gives water a cloudy to green color. Yellow/mustard algae looks more like powder or dust and is on pool surfaces including sticking to walls and usually starts growing on the shady side of the pool. Unfortunately, pollen and dirt are often mistaken for yellow/mustard algae, but the former do not continue to grow if put into a partially covered bucket. Algae tends to feel slimy while pollen and dirt usually don't (though pollen can feel, well, strange). A definitive test is to look at some of it under a microscope.

The reason I said you probably have yellow/mustard algae is that you said it was forming on the walls. That usually doesn't happen with pollen/dirt nor with green algae.

Richard