In addition to what Todd said:
If you have had freezing conditions recently, has the neighbor's sprinkler system not been winterized - could be it froze and broke a pipe - especially if they are new neighbors and don't know about having to shut off and blow out sprinkler sytems for the winter.
Also check if neighbor has changed the location of drain pipes or downspouts that could be running more water your way, or a broken off or stuck srpinkler head that is pouring out water in one place rather than distributing it. Walk around his yard and yours and feel for mushy spots indicative of shallow groundwater. Excessive green or luchness of grass might also show a trail to its source.
Do either of you have a septic tank/leach field that might need servicing ? Sewage leaking would almost always make for much lusher grass where it is coming to or near the surface, though in dry country just more water can do that too.
What is elevation difference, if any - does ground slope from your yard to his or vice versa ? If neighbor wants to cooperate, dig a couple of holes on each side of the block wall (saving grass divots for replanting if in lawn) - one a couple of feet away, one maybe 5 feet away, and check for how wet it is in each hole - may be significantly wetter on one side than other which might give an idea of a directional trend to the wetter side. If water wells up in the hole(s), if he has a chlorine test strip kit for his pool, you could check for chlorine content with a test strip, and then compare to tap water - might show if it is pool water or tap water, though depending on how far it is percolating the ground could have neutralized the chlorine before it reached the test holes.
If all the ideas fail to find it, and it worries you or you think it will grow or weaken the wall foundation, you last resort would be to contact a geotechnical engineer who could come out and dig shallow test holes and survey the water levels and possibly dye trace it, tracking it back to its source, but that will cost you probably $250+.