Horseshoe crab competing with Goby for food?


Reefing newb
I just purchased a Diamond Watchman Goby because I wanted to keep my tank as clean as possible. The first invert I purchases was a small horseshoe crab and I love him. He's certainly let my favorite creature in the tank.

My question is, in a smaller tank (20G long) will these guys compete over food?

The horseshoe crab often stays buried under the sand and doesn't clean it as I had hoped. I just don't want him to starve.

What should I do? Trade the Goby and get hosed at the LFS?
dont know if my yellow watchman goby is much different to your diamond in behaviour but it eats brine and mysis shrimp that i put in tank for my bangaii.i have a cleaner shrimp that goes mad for mysis so there is competition there between my goby, shrimp and bangaii not to mention my hermits etc.all doing well in a 10 gallon.