Quote Originally Posted by JimK View Post
I get the same thing sometimes. I've read that it's from body oils, lotion, sunscreen, etc. I do notice it more if someone using sunscreen has used the pool.

It is odd/interesting it only occurs in the skimmer.
It tends to be more concentrated in the skimmers and therefore easier to spot against the white background. All fats/oils and body fats/sweat etc are a source for food for bacteria and mould (mold) and usually do form a tide mark ring. The reason I suggested a mould (mold) is that light gets in through the skimmer opening and generally only hits one spot at the back of the skimmer as in your photos so growth commences there.

FBU, interesting, can't make a suggestion apart from the bromine bank gave a higher residual disinfection level, only a suggestion.