Ball joints can spooge out grease at any time in their life cycle. Leaking out a small amount of grease by itself has no immediate effect, and also has nothing to do with the internal mechanical 'health' of the ball joint. As long as most of the grease is still in it, it should be fine. Of course, if it's a lower ball joint and it's been on the car for 20 years and 100K or more miles, and especially if it's seen a large amount of rough duty, it is a possible danger point that you should look into replacing sooner rather than later.
Kind of like CV axle boots- the rubber can leak a bit, or have a hole torn in it, and the actual part still functions fine for quite a while.
If you will be driving through a large amount of flying sand/dust, or submerging it in water, especially dirty water, then I'd think about having it changed soon after.