Hi, Gina,
I do not think that is mustard algae. I think it is simply regular dead algae. Mustard algae shows as a smooth, even, yellowish layer on the floors and most always the walls. You show clumps gathered in the bottom of the pool and that color and it's clumping habit is most always dead algae.
SLAM your pool. You must, brush, vacuum and keep the chlorine at SLAM value until your pool is crystal clear and you can pass the OCLT
Duraleigh, this is pretty much what mine looks like too (from the copper/algaecide thread). Except mine gathers along some of the bottom seam of the liner between the floor and the wall. I never have it in splotches i the middle like the OP does.
I did spin my eyeballs to change the flow of water from clockwise to counterclockwise to see if that would help.
TC 12
FC 12
CC 0
pH 7.5
TA 70
CYA 70ish
Water temp 87 (heater has been turned off)