How do you clean the rubber around the rear window and prevent this?

I've had this happen on other vehicles. This time, I've been trying to keep the truck clean, but it's happening again. The rubber around the rear window is turning from smooth to rough.
How do you, if you can, repair this? How do you, if you can, prevent it?

 

Never seen this before, you parking under some kind of tree and it's dropping sap?
Some kind of rubber/vinyl protector should keep it clean. Good luck

 

Thanks. No trees either at home or work. Maybe it's the environment.

 

Use a magic eraser on it.

 

Do you live near jack daniels?
https://youtu.be/aKHihLgVvT4

 

Clean it up with a decent cleaner and keep it lubed up with 303 protectant.

 

It's mold or mildew. Vinegar should clean it off, then use a protectant as TJ said.

 

I will try these methods this weekend and let you know. Lynchburg is about an hour southeast of me.