Vacuum Pool Cleaner

It’s algae, specifically what’s called mustard algae. Second hardest to eradicate after black algae.
Your number one priority is to get your chemistry balanced. If you don’t, it’ll just come back. Once you water is balanced (don’t forget to make sure your total alkalinity is good, it’s the most important and also the most overlooked) vac your pool to "waste" or "drain" if this is a sand filter. Go for the algae spots first. Vaccing on waste bypasses the filter and puts those algae spores out the back wash line never to be seen again. If you do it on the "filter" setting, algae is small enough to work through the sand bed and come back out the pool returns. Work quickly, you’re removing water from the pool. If you’ve managed to vac up all the visible stuff, set the filter back to "filter" and continue for the rest of the pool. If this is a cartridge filter pool, you’ve got no choice but to vac the algae onto the cartridge. Same with a DE filter. Just clean off the cartridge when you’re done. Algae is too big to make it back to the pool through these types of filters. Depending on how much was vacc’d up, if this is a DE filter, you may need to clean the elements and re charge the system with DE.
Whatever algae is left, after your vac, attack with your pool brush to get it into the water column for the skimmer to deal with a bit and to expose the live algae spores still on the bottom to chlorine and algacide.
Keep your chlorine levels up to 1.5 – 3.0 and add what’s called a poly quat algacide in an amount applicable to your pool volume.. It’s expensive stuff, but it works. The cheaper alagcide containing dimethyl…. ( a word so long I can’t spell it) is just junk. It rarely works and is a waste of money.
Follow the above and your pool should be algae free in a day or two. If you have any pool side plants that are hanging over the deck into the pool, move them back. They contribute to algae issues.