I had it happen to me a couple of days ago. I sent support an email to see what was going on. I've played at Stars for awhile and had never seen nor heard of that before. And it really freaked me out when I asked my table about it. I thought everyone at the table got the message, it was just me. Which made me freak out a bit, so I sent support an email to see what the deal was, below is their reply. I feel it's a really good thing they are doing!!!
Hello Don,
Thank you for your Email.
The prompt you received is a legitimate part of PokerStars' robot player
detection and prevention system, known as a CAPTCHA. CAPTCHA is an
acronym for "Completely Automated Public Turing Test to tell Computers
and Humans Apart".
This system helps ensure that PokerStars is as free of "bots" as we can
make it. The system has been designed such that most players will
experience a CAPTCHA very rarely so as not to be intrusive upon your
game play. The exact criteria for who receives a CAPTCHA and when
cannot be divulged.
We ask for your cooperation in completing the CAPTCHA whenever it is
presented to you, in order to help protect the integrity and security of
our games here on PokerStars. Failure to complete the CAPTCHA when
prompted may subject your account to unnecessary further CAPTCHAs or
more careful scrutiny of your account.
Good luck in your games, and thank you for playing on PokerStars.
Regards,
Balint
PokerStars Support Team