Protein skimmer - what SHOULD I be seeing in the waste cup?

So I've been running the protein skimmer...with only 3 relatively
small fish, a few crabs and snails, and a cleaner shrimp, and want to
make sure I have it going right:
The skimmer has all the bubbles all hte way up to the waste cup, but
it doesn't ever seem to actually be flowing IN to the waste cup, no
matter if I open the valves all the way or not. That having been
said, every 3rd day or so I clean out the cup because there is a
coating of blackish/greenish crud on the sides of it...smells
horrible, and there IS a little bit of water with it...but I guess my
real questions are:
1. Should the bubbles be overflowing up into the waste cup?
2. What SHOULD I be seeing up there when I clean it out?

Mitch
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