jolson10450,
[welcome]
Horseshoe "crabs" are very cool animals. Unfortunately, these close relatives of spiders dig through the substrate to find food and eat any creatures they can find. They quickly depelete the fauna of the sandbed. I imagine the 57lbs of LR you have covers much of the sandbed, also. I'm not sure if they readily accept dead foods like chunks of shrimp, etc. Another thing to consider is a fully grown adult is about 2/3 the length of a 40 gallon aquarium.
I think they will eat any soft bodied organisms they find on or near the substrate. I'm not sure if that includes corals or not.
HTH,
Kevin