that could also be white powerdey mildew, but cant tell, either way some neem will help, but some sulfer soap will kill it imidiatly, just be carefull sulfer clogs stomata, and can caseu stunting if used to an extream, but I agree preventative with neem is the best for success. and when going flowering only neem mabey the first week or 2 before budsites start to devlop buds then stop spraying