I always dip my rock in fresh R/O water for a minute or two to flush the critters out, alternatively you can do it in a hyper salinity dip.
Might be an idea, before you put the substrate in, to put some meaty fish food into the tank and see if any escape the rock to go get at it, then catch em and net them out. You might not have any, i am cautious with mine as i hate bristle worms.