Previewing is, indeed different: all about previews in After Effects CC 2015 (13.5)
That being said, the green (and blue) bars are supposed to be there. There are some bugs...as you've noticed.
After Effects CC 2015 is the beginning of a HUGE change in how After Effects works. Notice how I said it's the "beginning"? Adobe is re-writing the core of the application to give it a much better architecture. This is good news for the future, but it does mean there are some issues to slog through in the meantime. CC 2015 is like a beta preview of parts of the new architecture. As you work with it, I'm sure you'll notice that it's much more responsive when you're messing with things in the user interface. That's because the renderer and UI are running on separate threads. I love this. It means I can move stuff around and it's all smooth and I don't have to wait for it to redraw the frame while I'm trying to interact. However, in 13.5.1, there are numerous bugs that abound beyond the intended features not available.
The good news is that we are very few days away from a significant update to CC 2015 that fixes many bugs including ones involving the cache indicators, if I recall correctly. The update is meant to arrive "at the end of this month", so that means less than two weeks away!
If you are experiencing issues with the new architecture that make CC 2015 unusable for you, you're not alone! Happily, the Creative Cloud app lets you install CS6, CC, and CC 2014 as well! See this page: How to Find and Install Previous Version of Adobe Apps in CC 2015
You can turn that green bar off or on using the timeline panel preferences. You can find them right near the comp name in the timeline. The little icon looks like three straight lines one on top of the other.
Strange I created a new document with a simple footage file and it shows
The initial project is very complicated, lots of 3d layers etc. Could it be a comp setting ?