Derrik--we have similar staining in our gunite pool and I attribute it to lawn chemicals. Back in the spring my hubby was broadcasting lawn food granules and though he SAID he didn't get any in the pool , the very next day the stains appeared. They have been there ever since and no amount of scrubbing, etc. will take them off. (I also noticed similar staining of our concrete driveway and sidewalk. ) Hopefully they will eventually fade. (The upside is that we have a very attractive green lawn. )