Squishy stomach

[Diane R]

View attachment 48176
I checked her litter box just know because I saw that she was going restroom. And saw this is it sludge or mucus there is also the poops on the side with white on it can that be dry sludge.

The white is probably just excess calcium in the urine which you don't need to worry about. Sludge has the consistency of toothpaste, can't tell from the pic. Take a sample to the vet?