Nudibranch, that is. I believe these are endemic to Hawaii. In Indonesia I saw some relatives that were golden/brown.
And no touching things down there in general. But a particularly good idea not to touch these. I’m told they eat hydroids and use the stingy part in their blue frills to sting things that touch them (and try to eat them).
Dive site: Molokini – Reef’s End, Maui, Hawaii