You might try acetone on a cloth or paper towel. I use that to get scuff marks off my fiberglass and mold/mildew off my outdoor vinyl cushions.
As for Turtlewaxing your tubs, be careful! Be sure to have good traction strips on the bottom or you're inviting a slip and fall....and....if that happens and they determine you waxed the tub, they will also determine that you caused the fall, and you know where that's going to lead.
I use a polish (Marine Finesse-It II by 3M) on mine rather than wax, applied with a variable speed polisher and a very soft foam pad - wool pads are too abrasive and create too much heat (friction). Wax comes off with detergents and has to be re-waxed more often. The polish lasts a year. And LOTS of traction strips. Finesse-It is good stuff. I was able to ressurect 28 year old Imron paint (over fiberglass) with that stuff which saved me from a $65,000 paint job!
On that stain, if all else fails, maybe try Whink (little brown bottle in the grocery store where the laundry detergent is) which is a rust stain remover. I use that a lot on my fiberglass surfaces and it works like magic..