Can you just take the top off the cup and let it flow back into the tank? You don't need a skimmer if you have plenty of aeration in a new tank. Skimmers take out a lot of the seeding bacteria and micro organisms that will come from the sand and rocks. You want those larval organisms and pods in there if you intend on a living reef system. If you just want a sterile FO, then it really doesn't matter. But, like Alex said, it takes time to break in a new skimmer. It may foam or do nothing at all at first. Let it flow back into the tank if its real thin or you'll be dumping too many trace elements and living organisms out along with that thin stuff. Sounds like you already dumped a lot of the micro organisms in the live sand anyway. Thats what you get in that cup until it seeds out and colonizes.