The crab detached them from their normal habitat, in order to put on its own back. The crab may be the one to benefit most.

With the seaweeds, it is clear that the crab is damaging the seaweed by cutting off foliage and clothing itself in them. Those seaweed fragments will soon die, and the crab will have to harvest more. The crab benefits, while the seaweed does not, since it has to grow new fronds to replace the ones the crab hacked off.

It is similar to Adam and Even hacking off leaves to produce clothing, the tree is damaged and does not benefit from the one-sided, non-symbiotic relationship.

This is not a matter of seeking out habitats but rather of modifying one's exterior.

Hong Kong foot is not a good analogy here.