LTO Tape drives after extended use will request cleaning.  You can purchase any brand of a Universal LTO Cleaning Cartridge as they're all of equal quality and all can be used about 50-times in total. See at the bottom of the article for a current list (as of July 2018) of cleaning cartridges.

Please DO NOT routinely use a cleaning cartridge under the assumption that you're performing regular maintenance.  Each time you use a cleaning cartridge it degrades the read/write head, think of the process similar to sharpening a pencil.

Only insert a cleaning cartridge when the tape drive requests to be cleaned, and if you've cleaned the drive and the light is still ON, please STOP using the drive and submit a support ticket for service.  

In some cases a cleaning light can be triggered by inserting a dirty or bad data cartridge.  It is a good idea to always try a different data cartridge if you're having cleaning light issues often, as you should rarely need to clean your LTO tape drive.  

Here are some cleaning cartridge part numbers:

Quantum - MR-LUCQN-01

HP - Q7978A

IBM - 35L2086

Fujifilm - 600004292

Sony - LTXCLWW

Oracle - 7063645