the public, smartware & timemachine shares are all public by default, you can change the smartware & timemachine shares but noy the public share. I don’t use the publlic share because of this, hopefully you can just tell people on your LAN to not use it either
you can disable public access to the Public share with a simple trick:
Change the share name of the Public share to something different and click save. An error will pop up showing that it’s forbidden to change the name. Just click OK and the option is not greyed out anymore and you can disable public access
What device are the guest on? I haven’t seen anything like the image you are showing, on my Windows PC’s.
I think the key here is you say when they login, I would not give anyone my network guest login information if I did not want them to see what is on my computers or My Cloud.
Maybe someone using the same computers or devices as you can tell you what to do. I havn’t found a way for anyone to sign into my My Cloud as a guest.