You can use a power washer in conjunction with part bleach part water. All the hotels around the beaches in Florida swear by it and use it all the time without any damage to the plaster.
Three ways: chemical agent that "bubbles" up the substance for cleaning; wire brush and elbow grease; or power/pressure washer. With all options take card not to remove the plaster.
There are various methods used to remove mold in moulding process. One commonly used process is Cope and Drag process. You can use this method to remove mold from chimney.
If you are wanting to remove this type of molding your will want to buy a remover. Once you have got the remover you should apply it and let it dry then remove.
