Substance coming out of bath water?

[John W.]

Just finished rehabbing a house. Tenant moved in over the weekend. I received a text tonight from the tenant with a picture of what is coming out of the bath tub. Looks like chalk/sand from what I can tell. You cant really tell until the tub is emptied, this stuff is left. Anybody have any ideas? So the house has been sitting empty for 4 or 5 months. Maybe something in the hot water heater that needs to be flushed out? If I knew how to load the photo, I would.

Thanks for any help you can provide.