If you were told that your pay grade was a 6 and you accepted the position on that basis, then you accepted the pay grade along with the job. Do you know it if was actually a 7 when you accepted the job or have they changed it since that time.
In any event, talk to your supervisor and point out the fact that you have discovered that your job description indicates that your pay grade should be a 7 and you would like to request that they increase your salary accordingly. If they are amenable, you might ask if they would consider making the raise retroactive to either your start date, or the date when they upgraded the payscale for your position.
You must realize of course that they can fire you for any reason or no reason because, unless you have a written contract, you are an at will employee. If they take exception at your asking for a raise, they don't have to have an excuse to let you go.
(If you have a contract for your job - you might go back and check to see if it indicates what you pay grade should have been when you were hired.)