Gritty stuff at the bottom of the pool overnight, mustard gas or big sand problem?

[summertimesadness]

First time pretty decent chunks of whatever that is, was at the bottom of the pool.

It's not slimy, which leads me to believe it's a sand issue? (I was able to brush that up to the shallow end to be vacuumed up)

 

[Mdragger88]

That appears to be sand or debris.
You can try to attach a skimmer sock to the return(s) to see if you catch anything & prevent more from entering the pool. If you do catch some you may want to inspect your sand filter components to see if you have a cracked lateral or if the sand is at the right level. What model is your sand filter?