Can anyone help tell me what this dust/ powder is? Is there something burrowing into my wall?

Hi I'm not sure whether to post this here somewhere else... I'm in a stone house in the UK. Our garage is an extra addition, built onto the side of the house. In one part of the garage, there is a constant flow of something that looks like stone dust/ grit that falls down. I clean it up and within a day or two it is back. It is only on this side of the garage along this wall.

Initially I thought that there was some creature burrowing in the stone as there are a few pits/ holes in the stone but a pest control guy said that it is likely water trickling down that is bringing stone with it. Whilst the dust does feel moist, I have never felt the stone wall to be wet -the roof looks like it has leaked in the past but again, nothing wet anywhere below. Happens all year round.

Any ideas? Thanks!