asked Dr. Ron, and here was his reply.
The animals are elysiid sacoglossan slugs. These are herbivores that generally eat some type of green algae such as Bryopsis, Caulerpa, or Codium.
It is difficult to tell just which species of elysiid they are. There are a lot of species and when they are small, like these guys, they tend to look alike. If you want to try to find which species they are you can check out The Sea Slug Forum. In any case, they are beneficial herbivores.
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Cheers, Ron