Growing Coraline

Hey guys, I have owned a 10g nano for almost a year now and am really enjoying it. Since summer coraline algae has started to grow at a noticeable rate. My water parameter's are okay with my nitrates a little high around 30-40 ppm, doing weekly to biweekly 10% water changes to lower it. I didn't start to notice it's growth until I left the filtration cartridge in the filter for a couple months, I heard that can help house beneficial bacteria and it seems to really be working. So here's what I have for you guys if you could help...

Is it true what I've heard about leaving the filter cartridge in?
Why has coraline started growing so fast?
Is it good/necessary?
How do I speed it's growth?

Thanks guys!