This comes up most every summer with forum member's ponds, including my pond. It is often very difficult to diagnose over the web, but the following guide should help you narrow it down to a couple options...

https://www.townofchapelhill.org/home/showdocument?id=28866


My gueses are...

Euglena and other flagellated algae (page 10),
Diatom Blooms (page 11),
Protozoan Scums (page 15).

Pay attention to the cyanobacteria pages and make sure it's not any of those...they can be pretty nasty.

Fish on!,
Noel