Strange square in lower-right of screen when looking at water from close range.

I have no idea what it is.

http://cloud.steampowered.com/ugc/614973792578470574/46F36B82C8B6C6DE41D08FB55E3753D79B849582/

Is there anyone who knows?

Everyone has that.It’s due to an Update released by Valve so it’s nothing to worry about.