I can't tell for sure, but it looks like you have a badly wrinkled area of the liner (possibly from a leak eroding the surface UNDER the liner) coupled mustard algae that's accumulated in the wrinkles.

Mustard algae will brush up, at least somewhat, into a greenish or yellowish cloud. But, the only way I know to leak-check a location like that is with SCUBA. Some guys can hold their breath -- but you have to be VERY still to check leaks in a location like that, and even those who are good breath holders usually can't be that still.