What’s going on with my cactus? Help

Lately my cacti have been showing some discoloration with some white-ish film and not sure why? I one water once a week and sometimes once every two weeks, I’m using a non inorganic mix of pumice, limestone with some Jacks Gritty Mix. They get sun but not direct, I have them underneath a window screen mesh to diffuse the light a bit and on rare occasions I move the screen so they can get some direct light for a few hours. Are they not getting enough water? Too much water? Not enough nutrients?