Hydrocal won't adhere to foam, but this will:
Get it at any building center, and also pick up a roll of metal window screen material while you are there. Metal screen...usually aluminum these days...will fold, bend and crumple and hold it's shape, while the plastic screen won't.
Apply the adhesive along the edges (top and bottom) in a bead...and maybe the ends too if needed. Polka dot globs for other large areas. Stick the screen right down on top--edges oversize, trim them later--so that the screen pretty much lies flat on the foam...or perhaps slightly off the foam surface. Smooth out adhesive that squeezes through the mesh with a finger (or glove...or baggie over your finger, etc) so it's smooth on top--it cleans up with warm water.
After it dries and you trim it out, apply thick body latex paint until the mesh disappears--one or more layers--and your chosen ground cover on top. I suppose you could try a plaster coat too, but I would be cautious about too much too thick. If it's too heavy a coat it might pull the screen mesh off.
You could also use it to fasten rock castings directly atop the foam without the screen at all...I think. Haven't tried it myself though.
Re: The edge beads. After you trim the screen, you can also apply another bead or two and smooth that out with a wet finger as well, until it meets your own "scenery edging standards." Then paint, and paint again as needed.