making a nest or being a pest?

Hi everybody. I am nine years old and have a six gallon acrylic tank with two mantis shrimp. One of them is chipping away at a giant clam in there. is he nest building or is he trying to eat the clam?should I take the clam out? Oh yea i got one more question.
How often do I feed my mantis?
Your mantis is probally very hungry and trying to eat the clam. What are you feeding it and how big is it? You should feed it every 2 or 3 days depending on what you are feeding it. look here-
mantis food

mantis food

Thanks death shrimp99
I have 2 saltwater gobys 1 blue legged hermit crab and a snail and they havn't eaten them - I thought they would. sometimes I put a silversides in, but they don't eat it, but the brittle star does.
I looked up that link and will try soem plankton tonight.