since its rigid and brittle, its most likely an epoxy. the only solvent i know of that will dissolve it will be methylene chloride. hard to get, carcinogenic, but good solvent for this. just make sure you wear silicone gloves (get a bunch as it can dissolve certain silicones too) and a full face respirator with a VOC cartridge or two in it. you can get it at your local plastics supply house or fabricator, or at a boat supply store (its used sometimes for acrylic and fiberglass work)
heres the rub - if its an acrylic tub, it will dissolve the acrylic too, so go slow and in a controlled manner. you might be better to mechanically abrade away what you can first. even if you chip or scratch the acrylic, it won't be an issue. if you haze the surface with some errant solvent, you can just polish it up later with buffing compounds.
if its a porcelainized steel tub, the mc will not do it any harm. just dissolve away. don't mechanically abrade anything because if you chip the porcelain, now you have a permanent rust starting location.