This is an interesting glitch, I don't know why this happens. As you can see in the image from the link below, it seems as though there are sponges everywhere. however, as you approach, they disappear. this is not harmful in any way, just interesting. http://imgur.com/E26vCaD
I believe you are referring to the end stone. As you can see, there are sponges on the layer adjacent to me
I don't think that's a glitch, someone placed them most likely with ./ps toggle off, meaning they are decor blocks instead of protection ones.