Retaining Wall Leaking Dirt
That small pile of dirt is an indication that moisture is behind your wall and is “escaping” by coming out from between the two walls. As the water runs out, it washes some dirt (fine particles) with it. 

If you look closely, you’ll notice there is some very small amounts of soil washing out between the joints of the “stack block” wall too. (If you notice, there is more dirt lower on the wall, because the water “escapes” at the lower levels where there is more pressure.) 

The wall on the left is made to be solid and not allow any seepage, but the stack block wall on the right is made to allow water to seep through the joints. 

If you plug the joint between the two walls, additional water and dirt will seep through the stack wall joints...you probably won’t notice the difference. 

Long term, water will continue washing soil out in order to reduce pressure on the wall. It’s annoyingly to clean up every year or so, but it’s not alarming.