Has the DPC (damp proof course) failed if moisture is showing above paint?
DPC is a water-impermeable horizontal layer, typically a type of plastic sheet, that you would find between two courses of masonry at the foot of a wall above outside ground level (e.g. one or two bricks above outside ground level). Its job is to prevent water rising by capillary action through the masonry etc from the wet ground below.

Obviously if the DPC is damaged or defective, it is usually not feasible to demolish the wall to rectify the DPC, so there are treatments which inject waterproofing material into the wall.

I am surprised that a vertical section of the wall corner has been treated - but I've not had this problem so can't be sure that is abnormal.

If that is an external corner, I would go outside and check that there is nothing outside bridging the DPC - like a heap of earth against the wall.

I would buy a damp test meter to determine if the discolouration is due to damp.

If it is the damp expert's treatment leaching through - and he didn't warn you this was likely - I'd consider asking them for advice and/or to rectify the problem.