Need help identifying this thing

[Mperry622]

I decided to turn a flashlight on the reef last night and check out the rocks. I noticed this guy a week ago and thought it was algae but when I went to get a flashlight he was gone. This tank was started with large amount of live rock back in April. Any idea if I should remove him or leave him be?
He looks like a brown piece of a lettuce with a head.lol

 

[ISpeakForTheSeas]

I decided to turn a flashlight on the reef last night and check out the rocks. I noticed this guy a week ago and thought it was algae but when I went to get a flashlight he was gone. This tank was started with large amount of live rock back in April. Any idea if I should remove him or leave him be?
He looks like a brown piece of a lettuce with a head.lol

My guess would be Elysia obtusa, a sarcoglossan (sap-sucking) slug. I don't what these guys specifically eat, but Elysia slugs feed on siphonalean algae such as Caulerpa, Codium, Acetabularia, Halimeda, Udotea, Byropsis, and Valonia spp.

So, unless you're running a macroalgae tank, it's probably harmless/beneficial, but you may need to offer it some algae to feed on if your tank doesn't have much algae in it.

 

[Mperry622]

I actually have three tanks connected together my main tank is 120 and I have a 65 on the left and a 55 on the right all plumbed to the main sump. 65 has nothing but Cheato and another leafy macro algea in it.
I don't clean the glass only in main display and I just have some rubble and about a quarter inch of sand and one powerhead.

Main tank every so often I steal a clump of the macro allergy leafy stuff my tangs and imperial juvenile go nuts on it. I do have a decent amount of allergy on half the rocks due to it all being live rock purchased back in April. The 55 has nothing in it it's just for water volume.

So I shouldn't be worried about him messing with the coral that's all I care about

Thanks for all the advice and response