I get what looks like algae. It’s on the bottom of my pool. It’s dark green. It’s comes back to the same spots after a day or two after I vacuum. This has been happening since last summer. I did shock the pool several times last summer. after the shocks I saw it back I thought it had to be just buildup of dirt and maybe leaves, but now I’m not sure. my pool is well balanced always and looks clear. Not sure what to do.