This crab makes for a far more convincing alien, though.
P. cuvieri is typical of members of the Homolidae family, the carrier crabs. They're crabs that carry stuff. Here we see one carrying a sponge over her head. It's only a sponge, so there's no need to overreact if she throws it at you.
Regardless of how big their pincers are, their hindmost legs are tiny in comparison. Akin to the Sponge Crabs, they have small hind legs positioned in such a way that they can put their legs in the air like they just don't care...
This appears wholly insufficient for any conceivable purpose
Presumably it helps defend or camouflage them from predators but it must be nice to have a bit of decorative company, too.