Unlike, say, dimethylhydrazine (brr, nasty stuff) mercury does not burn or decompose into harmless substances. It gets highly concentrated in marine life, because trophic chains go in one direction, and one predator eats large amounts of prey.

The fish humans eat are often many levels of predation up the chain, so concentration of mercury in, say, tuna can be troubling. Even if you inject relatively small amounts of mercury in the ocean, much of this amount is not dispersed but returned in seafood.

The amount of mercury used by spacecraft engines is likely (not necessarily) too small to matter right now. With cheap reusable boosters to orbit, it may grow quite a bit soon enough to matter.