I agree Bill. Though the pic's resolution is low, it appears very similar to the benthic sheet-algae that we discussed awhile back (blue-greens) - and which we (my area) seem to be encountering with greater frequency this season.

I haven't fully mapped the variables that coincide with its appearance. However, as would be expected, nutrient-loading appears to be a major commonality. Maybe some "septic-tank microbes" [sic] will do some good...

As for the concept of rainfall providing any relief: Based on our area's recent abundance of frequent rainfall - don't count on it offering any benefit. A hard rain will often cause surface mats of algae to seemingly "disappear" - but only temporarily - since it only serves to dislodge the gas bubbles which provide boyancy for the floating mat. The mats have quickly returned to the surface after collecting enough gas bubbles to regain boyancy.