yup thats powedery mildew and your on the right track sulfer burners are usualy used , however you already have the mildew so you need something to kill it and get rid of it. theri are alot of things for this, the easiest one to find is clled safer soap, fungicide or all in one. ususaly people burn sulfer about once a week for a half hour to a hour depeding on size of the room. to much sulfer can be bad to the plants too, so follow the directions.