It defiantly a nudibranch. I found it in my refugium. It's green and about an inch long. Sorry for the crappy pic. If I take it out of the water it balls up.
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These nudibranchs are popping up everywhere. Maybe you guys bought items from the same place? Maybe ya'll should collaborate.
https://www.reef2reef.com/forums/general-invert-forum/150250-id-hitchhiker-please.html