skimmer question

[e46er]

hi there

i have a 120G tank with a 40 g sump and a small bioload couple clowns 2 yellow tang a foxface and a diamond goby.
ive had my skimmer for like 3 yeas and it doesnt seem to pull much out into the collection cup it seems to just build up a thick gunk on the neck
its sitting in 8-8 1/4 " of water and its constant height.
i have it set right now so it produces about an inch of skimmate in the cup a week of gross stinky brown gunk but its not that thick if i touch the riser adjuster thing just a hair it overflows the cup in minutes. I was thinking of turning it to wide open and installing a gate valve to close it but im not sure that will be any finer adjustment than the standpipe.

or is that a normal amount for such a small bioload???
do other people get buildup in the next as well?
cheers