I suppose that could be a common mistake. I am just so used to seeing Pycnogonids and the nudibranchs on other things that I can readily tell the difference. The pycnogonids are easily taken care of, however, the nudibranchs are more difficult to get rid of.
Oh I know the difference. A new colony I have been QTing just got a dip and had over 20 nudis on it and at least 15 egg spirals. Grrrr.