I have about 6 steps that we made out of flat rocks at our house that go to our shed. They get some kind of light moss or algae on them that makes them very slippery. We've scraped them off before but they still get slippery. Is there any simple fix for this? I didn't know if there was a way to apply some kind of grit with a sealer to get traction? I know part of the problem is the rocks aren't course enough. Any suggestions?