Can not maintain a fc level

[Butterfly]

I agree w/linen that it does not sound like mustard algae.

You might want to try adding a little DE to the sand filter.
Here's a link to the how-to in Pool School: pool-school/add_de_to_a_sand_filter

Also, if you haven't cleaned the filter in a while, you might want to try that:
my-sand-is-channeled-how-to-fix-it-t7626.html

 

[Bama Rambler]

I bet it's just dirt. It's in the water and when you vacuum you stir it up a little and there's more that's suspended in the water and once you stop vacuuming that drops out and collects on the floor along with what you stirred up. It'll take several times of vacuuming to get it out.