food debris combined with bacteria forms a soft deposit.this on long standing becomes calcified and is called calculus.see a dentist and get your teeth cleaned.brush your teeth twicw daily in the morning and before you go to bed.use some chlorhexidine mouth washes.gargle warm salt water after every meal.you will soon get rid of the problem.
This condition is called tartar and is from not flossing.
The actual term is "calculus" and is common on the inside of the lower front teeth. Some people just seem to develop faster than others. The salivary glands under the tongue empty right onto the incisors and contributes to it.
Brushing better won't help with it at this point. You have to have it physically removed (instrumented) by a hygienist/dentist and then increase your brushing and flossing regimen to keep it off.