Recently purchased a home. On the bottom of a steel tub (i.e. non-plastic) is something to make the tub non-slippery. It's not those stickon vinyl items or residue from those sitckers. Rather the surface almost appears to be mortar-like and is in a set pattern on the tub. I tried scrapping with a single edge razor blade, but it doesn't seem to come off.

Can you suggest what it might be and how to remove it?

Thanks

Got a hunch someone treated your tub with a Non-Skid Compound such as this; Global Safe
If this is the case the compound's etched into the porcelain and can't be removed. Sorry, Tom

Sounds like you are in for a new tub or at least an epoxy paint job. Tom's right from what I read. Rust-oleum makes a 2 part paint for tubs and showers.