Mr Green is regulated by Malta and powered by Netent casino games. Both Malta and Netent do not accept citizens from the United States of America.
I sent an email to the Mr Green support to get a clear explanation. Here is their answer :
Unfortunately we do not accept the USA due to gambling laws and regulations in the country.
If you are a US citizen and living in another country then of course you would be accepted providing the country in which you reside does not have any gambling restrictions.
I am glad that you are enjoying using our site!
As part of the US gambling law, Mr Green preferred to step back from the US Markets. However, some online casinos still continue to accept US citizens. Check our top 3 US Casinos.
Technically, you could play to Mr Green if you signed up from abroad. For instance if you are in vacation in Colombia or in Argentina, you could play to Mr Green while you drink your Cuba Libre. However, you should not use a US address and an american credit card (from Bank of America for instance).
This is the same problem as "signing-up from abroad". You could use a VPN to play to Mr Green, but you would need to have a real address outside of the US (with electricity bill at your name for instance). If you have an aunt abroad, for instance in the Russia, it could work. Otherwise, in case you win a $7,000,000 jackpot, Mr Green could block your funds..
We know that Bovada is currently accepting US citizens. It is powered by Top Game software and offers a nice welcome bonus.