How do you know your blenny was killed by something? Did it show signs of injury? I mean before death as well, because once something dies, its free food for any omnivor/carnivor in your tanks, I've seen passive worms eat whole dead fish, even though they could never kill one. Because a number of "algae" blennies have been known to do poorly in captivity by eating the to much of the "wrong" type of algae resulting in bloating/death.