can you get a picture showing how the shells don't align? from your pic it looks slightly bearded, but perfectly fine. if the algae is from excess nitrate then the clam is probably happier from it (they like a bit of fertilizer), but if its from phosphate, it may be interfering with it's calcification. regardless, it doesn't look critical from that pic.
edit: looking at the pic again, i guess it is a bit strage that the mantle isn't visible where the algae is, but at the same time it looks like there's a healthy growth ring....can you get a top-down shot? and to emphasize the shell alignment issue, take another right after you gently blast it with a turkey baster so we can get a good shot of the shell.