Grass is a formidable foe and and I would think the only way is to weed it by hand and just try to keep on top of it.
I wonder if their is a biological way of attacking it, may be their is a nematode or mite that attacks this type of grass but leaves the plants you want alone.
I suggest you look into this possiblity, other than that the best of luck to you. Regards.

Ortho makes a product called 'Grass-B-Gon'- kills only grasses (monocots), not broadleaf plants (dicots).

I use a product to control weed seed germination and hand pull all the existing weeds. I think its worth it. Dichondra is one of life's best luxuries I think.

Poast is another herbicide used to kill grass in broadleaf ornamental plantings.