Dafuq is this frothing in my loop

Ek cryofuel, fully flushed my loop when first installed 6 months ago dis water, this caught my eye when dusting and upon taking it apart appears to be a froth like substance on the top of my Res where the liquid doesn't go to the top And yeh they're deep cool fans fuck off I know they're shit I got some stupid expensive Lian li uni's coming