I've had a Halloween crab for a couple of years in my SPS tank. I always get a kick when I see those little orange and yellow stripped legs sticking out. I have never seen him do any damage to my corals.
They are the same as the electric blue striped hermits and they are fine but they do get large enough to cause damage by dragging their shell around and flipping over rocks with their weight.
They also eat pixy dust and poop hopes and dreams... I think I've heard at one point or another that every snail and hermit species eats hair algae or red slime, but have yet to see any actually do it. I once saw black turbos tear through red sklime but I couldn't be sure they were actually consuming it. If you don't rely on on your hermits and snails to eat hair algae or red slime you'll be less likely to let the tank get that far and you won't end up disappointed. Get the hermit because it's cool and unusual.
Cool to look at, but big, annoying, bulldozing, pain in the assdroid, stupid XL crabs.
Reef safe. In my opinion, yes. Never had a problem. Only problem is knocking things over and killing the corals because they usually land upside down...