If you can get a picture, that would be immensely helpful. Otherwise, you may want to do a Google search for marine ich. It's usually pretty obvious once you've seen it. Also, what type of clownfish are we talking about, and expand upon "flops around in the sand". Does it look like the clownfish is moving the sand with his tail (Maroon clownfish will often do this), or does it look like it's rubbing its body in the sand, which could be a sign of a parasite.