I try to avoid metals out of principle.
Titanium should be safe, and I believe something like 300 series stainless should be, but I can't remember much about the various classifications of stainless steel.
I wouldn't put aluminum in, I think aluminum can be fairly toxic to a lot of things.
would tiles work for what you want? the back is usually unglazed/rough, so the moss should be able to cling to it, and it will sink. If you have trouble holding the moss in place, you could maybe put a piece of mesh over it, superglue the corners/edges, and let it grow through.