So I just watched some footage of the Terminator and T-90MS from IDEX and based on the the movement and texture of those areas I'm very confident that it's definitely rubber. None of the manufacturers so far seem to have moulded those parts like they would have been in service, but from a far distance you wouldn't notice so I wouldn't stress too much about getting it 100% accurate. It seems similar to the type of semi-flexible rubber on the T-72 that keeps its shape. Also, those parts of the fenders and side plates are close to the ground and track, and would be some of the first places to catch dirt and dust.
Here's the video if you'd like to see what I'm talking about, the way the side skirt areas move give it away. Even though it's a T-90MS, they both have a similar side armor makeup.
https://youtu.be/jGABgZRJoJ8?t=2m