Inflammation. That's the product of an immune system fighting something, whether bacterial viral or inflammatory.
Could be...also. I agree with dr. Villaplana that often "green boogers" represent some type of inflammation. If however, you do not have any symptoms--congestion, runny nose, sneezing, pressure/pain etc...It may be from you breathing in greenish/yellow pollens which get trapped in your nose (its job) and gets dried up as green/yellow booger. ..If you have bad symptoms, check wtih your doc. Good luck.