Bits of debris? They're Side gills!

Side-gilled slugs are close relatives of nudibranches but have only one set of gills on their right side. This one was found (after passing what must've been hundreds of them, overlooking them as black bits of debris) on the sandy bottom of the channel between the large and little patch reef along on Hantu West.

These side-gills are tiny little things, with the largest about the size of the nail on your pinky. They move really fast however and are good fun to observe!

Reference: Living Reefs of the Indo-Pacific: A Photographic Guide