I found one of these in my tank a long time ago, and it was bright orange, like Rob said, it was the Bright orange zoas he was eating on.
I noticed he was only on that colony I bought, and had a nest there little white eggs at the base of the zoas.
I didn't know much about how to get rid of them, so read up and found someone saying to give them a freshwater dip, so I used a toothpick to get the eggs off, then filled a bag with , YES!, tap water, threw in the colony of zoas and shook that bag as hard as I could severl times for about 15 minutes.
Then I took the colony out and swished it around in a bucket of saltwater for a few more minutes, then with it all closed up due to the aggrevation I inspected it as best I could for eggs and nudis....
Never saw one again.