They can be, but it won't be easy. It's mostly the oil, but barnacle and coral have also grown along the walls that would need to be chipped away, and the clams won't be too happy about it. The oil that covers the wall will also be difficult to clean, due to it being underwater as well as seemingly not normal oil. It's thick, sticky, and doesn't come off easy. Some may even notice there's some "magical" characteristics to it as well.

Once we have proof or plans of what to do, we can answer more...(oOooOoo)