I tried feeding feeder mollies once to a smaller smasher mantis I had. The mantis didn't touch a one of them. One of the mollies actually survived being placed in the saltwater and acclimated (they're actually a brackish water fish). The molly ended up living in the tank with the mantis for about 6 months. The mantis never did touch it, so I think you're right that your mantis will leave the frogfish alone. Nice tank and mantis. The frogfish is cool too.