yes you'll have to seal it
you can use polycrylic sealer or mod podge (some kinds may be sticky after drying so you'll probably want the outdoor or hard coat if you go with it)
i made one myself recently with foam, grout, cement color (to get a nice red rock color) , and mod podge hard coat here is an image
and here it is in use
and if you do go with the mode podge a tip brush it on in thin coats and after letting it dry for 20 or so min go over it again lightly with a brush to smooth out any droplets that ran or areas with more as otherwise when it dries you can get little white bubbles and such in the finish , and let it dry for about 3 days between coats (you'll only need a few coats anyway)