My experience with Great Stuff Foam is it might expand and contract just a tiny bit. In my builds using mortar mix painted on the surface the mortar will crack vs ridged foam. Acrylic paint is safe to use but whether it will stay in place I just can't answer. If its covered in silicone and peat fibers then it will remain flexible and you should have no issues.

For underwater use as long as acrylic paint is dry it should not wash away. I have used acrylic paint on the rock work for my water falls with no issues.