On my "wildlife pond" I tucked the edges of the liner under the grass, as I didn't want a brick edging or concrete slabs. This meant that a piece of the liner showed as the water didn't come right up to the grass.
What I did was sow long fine grass (the sort you get for shady lawns) onto pieces of weed control fabric (the black flimsy fabric sort) which I first covered in seed compost and kept in trays, moist. The grass germinated and I then had sort of grass covered mats which I then laid around the edges close to the pond. I also planted many sorts of small plants that like boggy conditions in the area, through this grass mat. After a year or so, the plants and the uncut grass, which is long and fine, have blended in with everything and hides the plastic quite well. Moss has found its way there too, but it is on the shady side of the garden so this is to be expected.
Thinking about it, you may be able to do the same thing with the fabric and moss, but think you'd have to start it off by getting chunks of moss from somewhere. I don't think you would be at all successful trying to grow it straight onto the plastic itself.
