Cleaning off black/brown sludge

[NYRANGER]

I was wonder how to properly clean off the black/brown sludge that appears on the screen. Is it ok to use your fingernail when cleaning since this is the stuff we do not want? Also, I have the black/brown sludge growing inside the squares of the screen, will this eventually turn green or should I somehow try to clean it out of the squares? Thanks.

[SantaMonica]

You need to clean all of it off every 3 days with a brush. Can't let black stuff stay on the screen.

After it starts growing brown, clean every 5 days.

After it start growing green, clean every 7 days.