How can I clean the non slip surface of my bathtub?

try a mold removing product.

Couple of different website ideas...

http://homeguides.sfgate.com/clean-slip-resistant-bathtub-27759.html

https://www.decoist.com/cleaning-non-slip-bathtub/

My guess is that it is a fiberglass tub?

The type of tub would dictate the recommended cleanser. Because fiberglass can be easily scratched, baking soda seems to be the less offensive of all the mild detergents listed in the resource below.

“For stubborn areas, such as bathtub rings, you can make a paste out of baking soda and a little water. Wet down the tub’s surface and gently rub the paste onto it with a sponge, soft nylon brush, or cloth.”

https://www.hometips.com/repair-fix/bathtub-fiberglass-cleaning.html

1:1 ratio of white vinegar and blue DAWN dish soap. Heat the vinegar in your microwave and combine vinegar and DAWN in a spray bottle. Spray liberally on a "dry" surface. Let set 10 minutes then give it a good scrub. Rinse and admire your efforts. This is my go-to solution for cleaning my shower doors as well. I do advise turning on your vent fan or opening a window because the vinegar smell can get a tad smelly.

Just wanted to agree with Loreana Shipley. Blue Dawn and vinegar is a fantastic tub cleaner. I live in an apartment and the tub floor always looked kind of dirty until I tried the vinegar/Dawn combo. Now it really looks clean!

Thank you all. White vinegar and Dawn have worked the best so far.

I love the Lysol toilet cleaner in the black bottle for lime and rust remover. I've used it everywhere in my bathroom. Just let it sit for a few minutes to do it's thing. Then take a soft scrub brush to get into the little indentations to get all the dirt out. I even like this product on the mold in grout in my shower. I used a new toilet brush in there to get in the cracks.