I'm sorry,
@KariannNor, you've just lost me. I think your hand-wringing is over the top here, and you have now stepped past the point where any useful outrage is going to be generated to change other's behaviours. Not that I expect that most people here who read your post are likely to need their behaviour changed in this regard.
I cannot tell what is on top of the can, and I'm not sure that knowing would make much difference. Your initial suggestion that it is disgusting I can understand. And it is disrespectful of the environment and one's fellow walkers and locals to litter in this way. But a concrete majone is not some ancient stone. It is a block of concrete, occasionally with the metal markers left intact, some installed just a few years ago, by the Xunta in Galicia. It deserves respect, not as some sacred object, but for the practical waymarking of a camino route that informs all of us who walk past it that we are on the path, and where we should go next.
I think you can expect any argument based on attaching any more significance than that to be ridiculed, which will completely detract from any useful reminder that anyone might need that littering, whether it be discarded food and drink containers or our own excreta, is to be avoided personally, and discouraged in others.
Note that a post suggesting that a majone is an ancient stone marker has been removed. I will edit this post again in due course, but I do think that anything that distracts from the underlying message is unhelpful.
I am also sure that there are many, if not all, of us, who see useful practices in our own countries that have made some, if not substantial, differences to the amount of litter in our built and natural environments. Maintaining the personal disciplines associated with these - leave no trace, packing out our trash, minimising our use of plastics, etc, etc, is a good thing, and it should be encouraged. We don't need a change in Spanish law to do that.
) has made a similar "deposit" on a big rock. 
