Skimmer problem

[Bosnianblood69]

I was wondering if my skimmer is not doing ok its a etss 750 powered out of sump by a mag drive 18 with 4 foot of tubing connecting skimmer and pump from sump. The water level in skimmer is in the neck of the skimmer about 3 ish inches from where it overflows into the collector cup so theres about 3 ish inches of bubbles. It produces green water and you can see black resedue
In the cup, is there not enough junk in the aquarium for it to be producing jumk and bubbles? Not enough power from pump to much power? Any advice guys? Thanks