This is what I'd suggest:
Mow the lawn!
Trim the hair grass to about 3-4 cm.
Even all the way across.
Do not worry, it's a grass just like the grass outside in a lawn and it will grow back.
Use some decent size scissors, hair cutting scissors tend to work well.
You can trim the moss to your choosing and shape.
You can also add some manzanita branches or similar twigs/branches coming out from the behind the moss mound.
Behind this, perhaps a nice fine needle red plant etc, say L arucata etc.
Up to you really.
You can also pull the front edge back so you slope the gravel front to back, you can also trim the lawn to keep it short in the frotn and taller in the rear to yield a similar effect.
What you do here is up to you as far as scaping, but simple tanks are very nice and show more discpline.
Regards,
Tom Barr