If you are planning on having plants in there, I wouldn't worry too much, they will hide/obscure quite a bit.
Most of the aquatic mosses will also grow terrestrially if they can keep their feet damp.
And even algae can help with camouflage and such.
If your concerned about how that little bit of foam will look, I think sanding/cuttting/roughing it up will work better then gluing stuff on to it. The esposed pores/cavities will be more suitable for algae and moss to grow on then the current surface.
Plus, you can still do the silicone and coco fiber methods (or anything similar)