Hmm, I've had something similar happen with my opae ula (completely different animals, but whatever). I'd recommend keeping the light in the tank very dim to slow the growth of the algae, while also providing plenty of places for the mantis to hide. If you are keeping the mantis in a tank without corals, make sure there is absolutely no nitrates or phosphates using extremely accurate test kits.
I'd recommend frequent water changes, a reasonably low water temperature (no higher than 78°F), plenty of water flow, and very nutritious food (LRS for example). What type of algae is it, by the way?
As for whether or not he'll make it, it's hard to say. It's one thing for crabs and lobsters to have algae, barnacles, etc. growing on them, but for mantis shrimp who are known for their excellent grooming habits and pristine exoskeletons... Has it spread to his swimmerets, gills, mouth parts, or clubs/spears?
I'd also recommend ordering some Fluconazole to see if it'll kill the algae on him.