Roof Nuggets?

I found this adorable (can you call moss adorable?) chunk of moss on my deck the other day. I think it must have fallen off of my roof at some point this winter and was buried under snow this whole time.  This little roof nugget had a twin brother too! I'm almost tempted to bring them inside and put them in a terrarium.

I didn't realize I had a green roof...

Does this mean my roof is getting old?