There's no single "rocket fuel". The Crew Dragon capsule which does the sea landings with people has engines that run on quite toxic hypergolic fuel (monomethyl

There's no single "rocket fuel". The Crew Dragon capsule which does the sea landings with people has engines that run on quite toxic hypergolic fuel (monomethyl hydrazine and nitrogen tetroxide - for reliability reasons), so for it there is indeed an issue if there's something not combusted. The main engines of Falcon which lift the Crew Dragon up run off of RP-1 which is essentially kerosene, which would be a pollutant if it leaks, but isn't especially toxic. And Starship, which we're talking about here, burns methane, which doesn't have any toxicity issues with incomplete combustion. The toxic hypergolic fuels are not going to be used on the Starship (or, indeed, any other SpaceX ship; even the Cargo Dragon variant does not use them, they're nasty stuff limited only to the Crew Dragon capsule), a leak there on landing would mostly be a fire hazard.