Denture brush(!) Nice good sized stiff bristles, bit of slant to the handle for a good angle for scrubbing. I got one first at the dollar store, later found one that had some rubber gripping to the handle (like toothbrushes have), that makes for a better grip. They're perfect size for grout. (And waaaay superior to regular toothbrush.)

For cleansers, I'm partial to Soft Scrub with bleach (the white kind, not the gel one). (Don't bother with dollar store Bab O knock off. It's total crap.)

Yeah, bit if a cleaning nut.
Those denture brushes are seriously great. You will now LOVE to do grout! Lol.

Oops, lol, what am I on about?! Maybe there is no grout(!) For fiberglass, shocking but true -- in many cases best results are to rub with a dry cotton terrycloth towel(!) On dry surface (NOT when shower is wet). If there is a big buildup, you may need to do a scrubdown with the softscrub (or even a baking soda slurry) first, then let dry before the dry towel buff. But you'd be surprised how effective just a dry towel can be for soapy buildup. (Also good on smooth tile and porcelain.