This is in fact a green algae that looks silver because it has a crystal surface and most light is reflected. Some light will penetrate it however, so that it can photosynthesise, which means that if you were inside it, you could look out but no-one could look in.

This might be why they call it ‘The Sailor’s Eyeball’ for it is believed that if you look into it closely, you can feel the soul of a lost sailor looking back out at you.