We grow Irish moss between our patio stones also. If you're absolutely sure nothing was spilled on it-absolutely-then pull it up and see if you don't have some type of pest in the soil beneath. You may have to use a dandelion picker to dig down, though Irish moss roots are pretty shallow. We had the stuff come back after four years of drought here once it rained really well. So something's after it underground (or something really did get spilled on it!).

Another thought is that some type of plant may be crowding it out. Generally though, I find that other plants grow right up in the middle of it.

I'd love to know what you find-may run into it myself!

Best,
Judy

um...you don't have a dog, do you? Or did a small child recently visit?

Dog urine