The pool is completely full, this is what btrfs is reporting, and it will be correct:
Pool: cache
Used: 931.00GiB
Free (estimated): 2.84MiB (min: 2.84MiB)
Free (statfs, df): 0.00B
Something is using that space, as for the mover not working shares use cache setting might not be correctly configured, or there could be other issues, you can take a look here for some common ones, but until you free up some space pool won't work.