We had the pipes off my buds bike the other day to pull the pully cover. We couldn't get the adjustment cover unscrewed.
We tried everything, rubber grips, washing it and our hands real well, everything!!!
I decided to grab his roll of Duct Tape and ripped a piece off long enough to wrap once around the screw off cover.
Grabbed it with my hand and it broke loose easyily. I found the Duct Tape is tackey and the grip on it is much better.
Try that if anyone can't get it loose.