Do urchins change the color of their spines?
If you believe it to be a nudibranch, then a fresh water dip with some lugols iodine will get the nudis off. You need to look for eggs though. They are little swirly white string like things that are normally on the zoo but can be laid on just about anything.
The process as I know it has you get some RO
water and match the PH
and temp of your tank water. Add 5 drops of Lugols iodine per gal
of water used. Let the zoo sit in the water 5 minutes and then remove. I will then inspect the zoo for eggs.
This dip is recommended for all new zoos before you add them to your tank. I had not done this but do now.
Look in the zoo forum on reefcentral. There are pics of the nudibranches and the eggs sacks. They go into more detail on the dip as well.