Brittle star and emerald crab- friends or foes?

So earlier today I put an emerald crab and a brittle star in my tank and they both immediately went under the same rock. Since the the crab will come out momentarily and then make its way back under while the brittle star just waves his arms out from the rock from time to time. At first I thought they might be trying to kill eachother but now I think they might be bestirs or something? Is this possible?



Is this typical behavior for either of them?