These pics reminded me of one I took several years ago of the
emerald crab I had in my old 10g.
It really looks like he's going for the Xenia to eat it, and I remember seeing him once or twice "grab" a stalk or two, but never did he actually rip it off and try to eat it. I think he was just busy chowing down on the nearby
bubble algae and got carried away or something. If yours is like mine, he'll devour all the bubble algae and might annoy a coral here or there, but will cause no lasting damage.