usually that type of back up may have occurred while someone was cleaning a drain. Possibly a neighbor, that is if you have common drains with another unit.Cabling a drain stirs up the sludge and sediments.
Do you know if that came up the drain or from the faucet in the tub? The reason I ask is it looks a lot like sulfur off pipes from a well when air gets into the pipes or someone was working on the system.
It's def coming up through the drain. I cleaned it
Out last night and a little came back this morning. I just notched that down the street they were doing some construction to pipes. I wonder if that has anything to do with it.
By the way, the water drains pretty good down that drain.