Analyze intensity of a specific region.

This is a fluorescence microscopic image of concrete sample. The darken areas represent the aggregates and the greentone area (the rest) is the cement paste. I'm trying to analyze the overall intensity of this cement paste (only in the cement paste) (without calculating the small green indications in the darken areas). Please could you help me to find a solution for this?

Also I'm looking for a matlab script to analyize more samples (similer to this one)