Wonder if the technique of getting wax off a tablecloth with an iron would work on rock? Put a few folds of really absorbant paper toweling or old rag towel over the wax and run over it gently with a hot iron. The towel should absorb the melted wax residue without spreading it more and some/most of the color stain should come up with it. Any stain left could probably be treated with a mild cleanser.

Denatured alcohol will remove it. Get a still bristled brush and some rags and apply and scrub. If it is latex it will be dissolved. If the over spray is a fine mist, you might even be able to use regular rubbing alcohol.

Doing so will open up a very drafty, quirky(in a bad way) and dirty (inexhaustible source of dust and grit) surface that was not intended to be showcased. If the plaster needs to come off in part to get new plaster to bond (undo previous poor repairs), that's one thing. You have no idea what you may be getting into. Casey

Probably a very old chimney that needs to be lined with metal (not sure what kind of metal) because of small cracks. That's what ba few friends ended up having to do on older homes they purchased. Definitely get someone professional in to look at. A few went with a direct vent gas fireplace (must vent to outside wall not chimney) rather than expense of repairs on chimney. My sister just got a new fire box replacement, one with fans and heat vent, for a efficient heat source rather than a more traditional one. He is very handy, built his own home and cabin. But he would not take on that project. Cost was around $7,000, which included whole new liner. It was replacing his fireplace bhe had for 25 years.