Why do you ask? Are you thinking about buying a nudibranch and wondering about compatability issues or did you see something that looked like a nudibranch feeding on your clam? If it's the former then don't do it. It won't eat the clam, but it won't likely eat anything else in your tank either and it will starve. If it's the latter, then no there aren't any nudibranchs that would pose a risk to the clam, but there are some large flatworms that look somewhat like nudibranchs that could attack the clam.