Found this on a website: Poor Tissue Expansion Sometimes a clam won't expand fully and may remain contracted for several days or longer. There may be a number of reasons for this happening, some of which are mentioned in this article: A pH that is too high or too low. Maintain a pH between 8.0 and 8.5. Too strong or too weak water current. Being downstream of a coral releasing terpenoids into the water. Move the coral or the clam. If more of your tank's critters are affected, use protein skimming, carbon, and water changes to remove the compounds from the water. If this doesn't work, the offending coral will have to be removed from the tank. Stinging sweeper tentacles from neighboring corals. Move the coral away from the clam. Fouling or over-growth of algae, bacteria or mucus. Siphon the stuff away and alter the water flow around the clam. Being irritated by a fish or other tank inhabitants. Excessive UV light. Move the clam lower in the tank and make sure iodide is being added in proper quantities. You may need to put UV shielding between your lights and the tank. Too intense light. Move the clam lower in the tank. I'll look into those ideas.