It is something that has grown fungus/mold on it. Just clip the leaf with it off and it will be fine.
When ever you like really. I usually do mine from my canister every 8wks. Really depends on your stock levels etc. Just remember to rinse it in old tank water and don't let it dry out....When should I take out my filter floss to rinse the particulates out of it? Or will that kill my bacteria?