What a wonderful find! You found what we call hypolithic crust or hypolithic microbial communities. They are really cool. The green things you see are cyanobacteria and eukaryotic algae that can establish underneath light transmitting quartz (that rock type you found). It's so fascinating. The rock collects water underneath on the bottom side which is embedded in the soil. And this water can not evaporate because the rock acts as a barrier, so it's like a little green house under there! And algae love it!So water is trapped under the rock, light shines through it, and voila! Life! Water and light. And soil. And bacteria. But basically, water and light! Yay!