HELP! lazy crocea

[elfstone]

i have 2 crocea. The bigger one is 4 inches, i had it for 1 month and it is fine until recently when i introduce the smaller guy (in tank for a week and doing fine)the smaller guy shut its shell and redraw its mantle very quickly when it is slightly disturb while the big guy hardly shut its shell and at most close it shell slightly. beside this problem, it has open much bigger, the injuried mantle (bruised when the LFS keeper drop it on the sand bed) is healing and the clam is getting brighter .

is my big guy just lazy and really comfy in my tank whil my little guy is still getting used to the fish shadow?