What is this thing

[jcannon67]

I just found a few of these in my tank and I really can’t figure it out but it’s slug like with frilly sides

 

[Tahoe61]

It looks like a Nudibrach. Have you introduced any new corals or Macroalgae, and if so what types?

 

[ISpeakForTheSeas]

Welcome to Reef2Reef!

That's an Elysia sp. Sacoglossan Slug, either Elysia flava or Elysia obtusa - both are herbivores:

if it's from the Atlantic, it's E. flava; if it's from the Pacific, it's E. obtusa

Elysia flava or E. obtusa - harmless/beneficial Sacoglossan slugs. They're herbivores, with E. obtusa known to feed on Bryopsis, while E. Flava is thought to feed on Cladophora.

If you don't have access to their preferred feeds, you can try some of the below or another green algae, but no guarantees they'll feed on it/derive adequate nutrition from it:

Elysia slugs generally feed on siphonalean algae such as Caulerpa, Codium, Acetabularia, Halimeda, Udotea, Byropsis, and Valonia spp

 

[jcannon67]

Welcome to Reef2Reef!

That's an Elysia sp. Sacoglossan Slug, either Elysia flava or Elysia obtusa - both are herbivores:

Okay cool, Is it safe to leave? When I discovered them last night there are at least 10-15.

 

[jcannon67]

It looks like a Nudibrach. Have you introduced any new corals or Macroalgae, and if so what types?

Yes I just introduced a bunch of corals over the last month. I added a torch, Duncan, frogspawn, favia, multiple zoas, mushrooms, and 2 hammers.

 

[ISpeakForTheSeas]

Okay cool, Is it safe to leave? When I discovered them last night there are at least 10-15.

Yeah, unless you're trying to grow an ornamental algae they might eat, they'll be safe.