The Recaptcha module has a feature called Mailhide that allows you to protect the display of email addresses until a user has completed a reCaptcha challenge in order to reduce spam. This error message is telling you that the module is not able to operate correctly meaning that your email addresses will remain exposed to spam.
The Mailhide feature relies on a PHP module called mcrypt, which has a function called mcrypt_encrypt(). Your server does not have this PHP module installed in a working fashion meaning the reCaptcha module is not able to encrypt data in turn meaning it is not able to send requests to Google's mailhide API.
To solve this problem all you need to do is install the mcrypt PHP module: