I just heard back from Wet Web Media. They're not exactly sure what the cause is, but are guessing that it could be my zoas/palys or leather coral since they secrete mucous. They recommended that I change the chemi-pure elite and purigen that I run in my sump every month (not every 6 months like I have been doing).
They also recommended that I could consider redox/ozone. I know nothing about redox/ozone; can you all clue me in? :question:
But, their conclusion was that as long as everything in my tank is happy, I don't need to worry about some slime in my sump.