if its moving that much it doesn't want to stay there...
I tried putting a curved rock in front of one of my maximas to help prop it up... it broke the rock and settled in between the cracked pieces...
Try putting a rock next to it to lean it on or under it to attach to.
What type of clam do you have?
It's perfectly normal for clams to "hop" around a bit. But they do tend to skip more when they are distressed. And for rock borers like crocea and maxima they don't move as much once they are anchored. But my 10" derasa and squamosal are sitting freely on the sand bed, and they all move a bit here and there every now and then, no more than 1/4" each time though.
SataMonicaHelp Beat me to it. But either way, if it's moving that much it's probably not happy where your putting it, generally it's either trying to get to better lighting or better water flow.
my derosa moves several times a month and is growing into a monster.... so i don't mess with it.I would think if it looks healthy and plump i wouldnt mess with it(i've only had this 1 clam though)