Butterflies and angels are my favorite fish I have been experimenting for years with keeping these fish in reef tanks and I can say from experience that the chances are very high that it will kill your clam. Keep in mind that the racoon only has to pick at the clam and this will stress it out enough to kill it.
Unlike some of the other hit or miss fish like most angels and some butterflies like copperbands racoons actually eat clams in the wild along with shrimp, snails, crabs, and SPS coral polyps. I consider them one of the most destructive fish you can put in a reef tank.