What did you get it with or on? Normally their color is a simple concentrate from the food they eat and many Elysia are mostly algae eaters.
Make a cool nano for him and put some things in there and see if he leaves them alone. He many hide in something but you have to really watch
with a lense to see if it eats the algae from the surface or more. I've had them from time to time and unless its bothering the macro in my refugium
I let this genus stay in the main tank. To be safe, if you choose that too, be sure you verify it does eat algae first unless its easy for you to get it
back out.