Lo siento on your sea slug's passing.
I picked up a couple of the Pacific ones (e. diomedea) over the last few months... first one stranded itself in Outer Space (i.e. the wrong side of the lid) & dried to leather two weeks after adding. The other did OK for a month and then got apparently snacked on by my fire shrimp.
They're pretty creatures, but seem to be cursed in smaller tanks, or get lost in larger.