Check that your waters parameters are in check.
The white film you describe is a symptom of the calms inability to lay down new shell layer properly.
as a clam grows it builds its new shell on top of the old shell .
To increase or lay down new shell material, a clam must accumulate inside its tissue or more precisely up in the underside of its mantel lip tissues ......all the required ingredients needed to make shell material. (stuff like like Calcium and Calcite)it stores this shell building stuff to the point at which its tissue is so saturated with calcium that the Calcium precipitates out of the tissue and onto the existing shell.
This new layer is like a ring on a tree trunk.
Each new growth ring is a historic record of the growth conditions during that time.
If the clam/tree is stressed or some of the ingredients are not available or missing , the new growth layer will be soft and thin and in the case of the clam not adhere to the old shell.
This syndrome is called " premature mantel precipitation " or PMP for short.
It can be brought on by a number of water quality short comings and or stress which might make the clam excrete its mortar too quickly so to speak.
