Rear X rod always dirty with black grease

I have noticed that the rear X rod of my M200 Plus always gets dirty of black grease during each print.
 
Even if I clean the rod removing the dirty grease and put some new grease, the rod becomes dirty of black grease again after just a few minutes.
This problem is present only for the rear X rod, the other rods look much cleaner, even the grease has a normal colour, just a bit orange after hours of printing.
 
So what is the problem that causes this? Is there any part that is getting worn out early?
 
I have noticed that the rear X rod of my M200 Plus always gets dirty of black grease during each print.
 
Even if I clean the rod removing the dirty grease and put some new grease, the rod becomes dirty of black grease again after just a few minutes.
This problem is present only for the rear X rod, the other rods look much cleaner, even the grease has a normal colour, just a bit orange after hours of printing.
 
So what is the problem that causes this? Is there any part that is getting worn out early?

There has to be some old, dirty grease somewhere in the axis block - it may come out from time to time. Anyway, it shouldn't cause any damage. Please carry out axes maintenance regularly and everything should be fine.

Thank you Marek for your reply.

Anyway it is strange that it can be “old dirty grease”, considering that my M200 Plus is brand new and I’ve got it only by 3 weeks.