It might be ok but if you can avoid it I would. If your doing a tank black out then I'd put the clam in the sump with light. Clams depend on light to get the carbon energy they need for repitory functions, etc. Its not like a coral going without light. Then you have to be careful with light shock from the clam being in total darkness for a couple days. Likely it will be ok but if possible, I'd avoid it.