I don't know how to explain it, but it looks like the clouds texture that is turned green that you see sometimes when looking at the sky. I had this problem before in older versions.
It appears to be at clouds height.
Linked issues
duplicates 1
Attachments
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Duplicates MC-6943
Outdated Java & Graphics driver:
Your Java version is out of date! Please update your Java to the latest version (1.7.0_51): http://www.java.com/en/download/.
When using a 32-bit browser on a 64-bit operating system the automatic version detection will only show the 32-bit version of Java, the 64-bit version needs to be downloaded manually: http://www.java.com/en/download/manual.jsp.
Please update your graphics card drivers following these instructions: Updating Graphics Card Drivers.
If you are using Java arguments to increase the amount of memory, please reduce it to the default 512MB.
Do not rely on automatic searches for driver updates.
Do not use Windows Update or Device Manager - Microsoft doesn't install OpenGL, they promote DirectX.
If you know the brand and type of your graphics card, go to the links provided above and search manually for the most recent driver for your card. The automatic detection tools of the card manufacturers are not recommended.
If all that won't help, please use the Mojang Support Center.