After searching through your thread to find your light (first page), then cross-referencing the nano box for the dual-nano one (2x13 LED pads, right?), I think you should be okay.
While clams do eat particulate matter, it'll be much smaller than anything that your NPS corals eat. They may filter the water a bit, but don't buy them for that reason. And do remember that they need lots of calcium and alkalinity, and especially more LIGHT. You have a good light and your corals have been doing well, so you should be okay.
Clams are kind of a pain though, and neither of mine have actually "settled" into their spots, one gets moved around now and then, and the other moves himself around a lot. They've been in the tank for about 6 months. I think they're just not happy because there isn't enough room for them to really find a perfect spot. But they're growing and open every day, and responsive to light changes, so I guess they're doing okay.