iPhone 6 weird dismiss message

Hi,

So I recently got a new iPhone 6 and after a few days of owning it, I randomly got a weird message on the screen- the screen went light grey and it had some weird characters like '?!!!!!!!!' and at the bottom there was a button saying 'dismiss'. Today I got another message with the same characters and the same dismiss button. Is this a virus? Or is it a glitch of some sort?

Thanks.