This is a common problem with my masks. But when I asked my LDS what caused it the answer was "Never Heard of this before"? He called the manufacturer and their answer was "Never Heard of this before"? I too can rub this stuff off with a dry cloth, but its back the next dive. So I sent an e-mail to the manufacturer and was advised to rinse the mask with JetDry a dishwasher product. I did it. DON'T YOU DO IT! It nearly ruined my mask. It took 3 months for me to get the stuff off my lens, meanwhile every dive it fogged. (I dive every weekend) I scrubbed the lens with toothpaste (didn't work), Softscrub 5 times (didn't work), fine sand (didn't work), I tried various mask defoggers, none worked. The final solution to removing the JetDry from the lens was a long rub with an automotive rubbing compound. That did it. I was about to toss the mask away and buy a new one when it finally was able to defog. Now I just leave the white chalky stuff on the mask and dive. If you don't like the white chalky look, buy a black rubber mask.
:bonk: