It is tartar which is situated below the gums. Whenever tartar is located below the gums, it takes greenish or darkish color. It will be hard and can't be removed by brushing and it has also caused some inflammation in your gums in your case.
Next Steps
visit a dentist, preferably a gum specialist to get a deep cleaning done. you may visit me (am a gum specialist) at my Care n Concern Family Dental Clinic too
Health Tips
if left untreated, the tartar will go deeper eating away your bone leading to weakening of your teeth and loss of teeth in long run