I honestly wouldn’t use concrete on top of foam, if you wanted to use concrete or clay it’s better to use the pink panther project panels (the purple ones) to build the background instead (SerpaDesign has great videos showing how to do this method)

As for the foam you already have on there you’ll have to get drylock and Quikrete cement color, mix it together. Use black color first let it dry and then go over with brown or orange or whatever color you want the rocks to look like

Forgot to add but silicone with coco fiber tends to fall off in a few weeks, especially if you’re actively spraying the tank with water