That is normal, the car is 6 years old and is being used. To clean the innerside of the gauge cluster glass, you need to take out the cluster and remove the back panel, electrical base plate and gauges. The cluster itself is "sealed" by stickers. You can easily get it out and clean it, but a little dust won't hurt the cluster.
I presume that it is only visible when the sun hits the cluster under a certain angle...

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)