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You may not need to clean it off the transfer belt - which kind of damages easily, and which you (besides that) are having difficulty removing.
If these are all photocopies, that have a line on them, and they are all made through the document feeder on top of the machie, then you may have a problem with the scanner glass used by the document feeder.
To test this, just place one blank sheet of paper on the scanner glass sheet, and photocopy that four or five times. See if the line fades away at all - or possibly, rapidly - while doing so.
If the line fades away by 4 or 5 copies made on the glass sheet, then you probably have dirt on the scanner parts. Let me know if that is the case, so that I may advise you further, as to how to deal with the problem.