You can't get it taken off, but the top 3 credit bureaus do let you add a statement to your report. In the statement you can say why something is on the report. So you can try doing that, just explain that it was a mistake that been forgotten, but as soon as you realized there was a problem you paid the bill.
Who knows how many businesses will be swayed by any statements on a credit report, but it can't hurt to try.
Here is the details for Trans Union, you can also add the statement to the other 2 credit bureaus:
How do I add or remove a consumer statement on my credit report?
Send your consumer statement or removal request along with your name, address, and TransUnion File Identification Number (FIN) from your credit report to:
TransUnion Consumer Relations
P. O. Box 2000
Chester, PA 19022
If you would like assistance in writing your consumer statement, please call us at toll-free at 1-800-916-8800, Monday through Friday, 8:00 a.m. - 8:00 p.m. ET, excluding major holidays. If you are a resident of Hawaii or Alaska, representatives are available to assist you Monday through Friday, 8:30 a.m. - 4:30 p.m. in your local time zone, excluding major holidays. If you already have a copy of your TransUnion credit report, be sure to have your TransUnion File Number available when you call.