Surface scum is usually a sign of either or a combination of the following:
1>low flow
2>poor or no skimming
3>major die off of live rock, corals, anemones, etc...
Skimming the surface with a cup or shot glass will not remove all the waste in your system, instead bump up the flow and do weekly water changes or add a skimmer. This surface scum can kill other corals as it will build up mucus which will choke out other corals and inverts, your fish won't be too happy as well!
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