It's probably best to assume that any mantis isn't safe. You might could post him in the sale forum--there are plenty of people on RC who love mantises and would give him a good home.
In a smaller tank like yours, a Meicanthus spp. fangblenny is an option. Two of the most common ones are the "canary" fangblenny (M. oualanensis) and the striped fangblenny (M. grammistes). They're pretty and very peaceful little fish, but also armed with venomous fangs that they use in self-defense. IME, even aggressive fish tend to leave them alone. I used to have a female clarkii who was so aggressive that she killed a rabbitfish, but she never bothered the canary fangblenny who shared the tank, even when he swam right past her toadstool coral.