That's due to damp, and I would guess it's because the fall of the paving next to it is running in towards it. You have an injected damp course which protects the brickwork above, but anything below will be rotting and crumbling, and needs to be repointed really. Best opening gambit is to put a small drain in or drop the paving at the other side so the water runs away from the house. The brickwork can be cleaned with a power washer but be careful as it looks like you have the old lime-based mortar and it will be very crumbly. With no intervention it could cause problems inside, especially if you have a suspended floor (which I think you have, looking at the pics); the damp gets in and rots the end of the joists Hope this helps