Coarse grasses such as Meadow Grass, Yorkshire Fog etc can invade your croquet lawns. There isn't a selective weedkiller that will only kill certain grasses. This leaves three methods of dealing with it.
Killing weeds is quite easy. All that is required is a selective, systemic weedkiller such as Weedol Verdone which is available from garden centres. Professional weedkillers require Pesticide Certification but are cheaper. Systemic weedkillers enter the leaf and then reach the rest of the plant through its vascular system. However it works best when sugars are moving through the plant and that is when it is growing. So the best time to treat weeds is in the growing season.
This weed gets a special mention because it can invade croquet lawns and spread quite quickly. It looks very much like moss but can be distinguished by the tiny seedheads. It can be very difficult to spot so you have to look closely. It is quite a bright green and forms a mat in the sward.
Treat with a systemic weedkiller but it will almost certainly need more than one application