Brown patches on bottom of unground vinyl pool. Chemicals are correct. Won't scrub and algaecide does nothing however it is some sort of algae. The only way to get rid of it is dump shock directly on top of it so it coats it. After 10 min, it's totally gone with no stain. Next day it's back in the exact same area. Pool company is baffled and "pool doctor" said he thinks it's coming through the liner not from the water (???) and only way to permanently get rid of it is next time I get a new liner they chemically treat the soil. Anyone heard of this or have any suggestions?