Help? Yes. An all-out cure? No. I've used it with success, for a mild PM issue, but as far as i know, the neem oil just suffocates the mold, and doesn't actually kill it. So, my advice, is to use it heavily, because I won't hurt your plants. It'll make them shiny and mite-resistant, but that's about it.lol
For more established PM problems, you might want to look into something more specific to fighting mildew. Also, keep the humidity down(40-50%, if possible) and the airflow up, which'll help alot.