What to use for sealing foam background?

I am making a foam background for my enclosure and I'll be using grout to make it rock like. But I am wondering if I need to seal the background.
I will be spraying the enclosure with water from time to time and grout isn't waterproof. Is there anything non toxic I can use to seal it but doens't give a shiny look like epoxy?

You can get silicone with a matte finish. It's what I use.