Removing Bathmat Adhesive from Tub

Goof-Off is an inexpensive solvent that won't etch your fiberglass tub and is totally safe on most surfaces.
We use it all the time (my husband uses it to remose the stickers from sheets of glass and other items he uses in his artwork)
It's fantastic.

Rubbing alcohol has taken adhesive residue off my bathroom sink after I removed one of those sink skirts. I'm not sure what type of tub you have, but you might want to check with the manufacturer before trying anything.

If it's a porcelain tub you can scrape off the residue with a single edge razor blade. I would not try that on fiberglass.