I think it is algae now....

[waterbear]

My understanding of mustard algae is that it tends to creep up on the sides of the pool. Your stuff appears to settle in and get trapped by the small creases in the liner.

You said the pool hasn't been used or vacuumed in a week which is a good long time for dirt and dust to collect on the bottom.



AnnaK

Anna, Good Catch!

Mustard will not just settle in the cracks but ususally covers the sides and bottom, particulalrly in the shady parts of the pool. If you brush it away it's usually back in a few hours. This is not what you have been describing.
Not vacuuming for a week will allow a lot of dirt to collect in the cracks. I suspect that is what is going on here.