Heh, I had a RCC in college in a community tank. He never ate any of my fish, but he did have feeding time memorized. He would run out, grab an algae wafer, and run back to his hiding spot.
I kept him completely freshwater, without anywhere to get out of the water. One morning the little bugger was crawling across my floor. Imagine waking up groggy eyed in college to a dust bunny skittering around.
Crayfish also go wandering (and ime are much more adept at escaping), so I didn't think much of it needing land to crawl.
I had a second RCC a few years later that lasted until I gave him away b/c I was moving. He spent a year completely submerged, molted several times, and even regrew a front claw. (I bought him at a discount with only one claw) In that tank I varied terrain considerably - large rocks on one side, with different depths of gravel on the other (55g). The crab hid out in the rocks, and would do the same thing the last one did. Run out, grab an algae wafer, run back. Never touched the fish, and he lived with 4 dwarf puffers, ghost shrimp, and few (2-3) other small things that never went near the bottom.