Pretty sure the clam is flat-out dieing. If you can't provide an animal with the proper requirements for sustained health and well-being, then don't buy it. It's ridiculous that you thought a clam would cure your "green water" problem when the water quality is, most likely, terrible. I would focus on improving filtration mechanically - or even chemically, before investing in a clam that requires high water quality and excellent lighting. Sorry, but a nearby window that experiences maybe an hour of direct sunlight a day will not suffice. Try a product by the name Al-gone - works wonders. Secondly, address the basics ie. Water source, feeding regimen, nutrient level control, filter/skimmer efficiency. Remember that algae growth thrives on sunlight (~5500-6500K) so if you have high phosphate/nitrate levels, you're gonna have algae. Trade the clam in for some store credit that you can use toward helping the cause, not making it worse.
I may have to resort to a uv sterilizer....
