Sea Hares for many reason seem to meet thier demise in the most unexpected ways. Ive lost 3 of them in my first year of reefing. First one just vanished, I searched hi and low for week for him and after still looked around when rocks were moved around.
The 2nd one vanished after becomming the size of baseball, biggest Sea Hare id ever seen cuz he slithered over the divider into my macro fuge and had an absolute feast for a week. He was dropping logs of poop all over the fuge so I put him back in the MT where he vanished. He was later found under a rock. Seems when my rock wall fell down he got pinched and stuck between two rocks and decayed there for months. Suprisingly the system continuted to thrive, including a Moorish Idol, while a large corpse decomposed. In fact theres still pieces of the thing embedded to the underbelly of the big rock lol.
And as I recently mentioned, the 3rd got some kind of 2 inch worm borrow straight through his head to the other side. Kinda looked like those gag arrows you buy to look like you were shot by Robin Hood.
I did buy a 4th yesterday though. This one is the black with blue spots kind with frilly horns or spikes. They move much slower but look cooler. I dont think these type have the appetite of the brown ones but he seems to love glass despite it being freshly mag-floated and placed right amongst the tufts of leafy turf I intented him to eat. Seems the great deal I got at Aqua Hut (3 for $10) on Emerald Crabs has claimed this small area and are trimming it slowly but surely.
Anyway, he'll probably find himself in the Mt soon. I think its a very cool animal even if you arnt in serious need of algae control. Will be keeping an eye on him daily this time and make sure there are no more rock work collapses
Sorry bout your other Sea Hare man. They are my favorite means of algae control but have a habit of dying in ways youd least expect.