Tonight I decided to fix one of many cracks in my old plaster walls. This one is on the chimney on the top floor (a floor above the nearest fireplace.) There was a leak at the very top of the chimney, above the roof, which I recently had a mason repair; I hope that this leak accounts for this particular bit of cracked plaster (it's certainly in the right place.) In any case:

I was clearing away some of the crumbly plaster, and about half an inch below the surface I hit what appears to be a brick that has gone completely to powder. (You don't even have to sc**** it to see that it's all powder; the ex-brick just pours out through the newly opened crack all by itself.) As far as I can tell, there's only the one brick in powder form, though alas I do not have X-Ray vision to see through all the plaster. But the stuff beneath the rest of the crack in the plaster seems solid.

Question: what do I do? Naturally, I would rather patch it up and wait until I have dealt with the other million things that my 1850-built house desperately needs before dealing with this. But I love my house, and if I need to get a mason etc., then I will. I just have no idea how important it is to deal with this -- it's not on the outside or anything, and I assume that the chimney is more than one brick thick ...

If I do hire a mason, what should I ask him to do?

Thanks in advance.