I'm sorry nobody replied to you sooner.
If the installation of Plesk you have installed has been kept up to date then there won't be anything to worry about.
The issue mentioned is something that was resolved back in September 2011, at least for Plesk 8.6 (and, I think, later versions -- check the KB for details). If you have run the Plesk Updater (via the panel iteslf) since then, and installed the neccessary updates, your installation will not have been vulnerable.
If your Plesk installation was vulnerable in January 2012, AND the bad guys had you on their radar, it is possible that they gained access to all the usernames and passwords set up on your panel (for Plesk, not Moodle), including the Admin username and password.
In February, the bad guys began to exploit the information they had, gaining access to as many Plesk installations as possible and planting various forms of malware on various sites.
Even if your Plesk installation was vulnerable, and even if the bad guys got hold of the plesk username and password, as long as they have not done anything since then, then you can protect yourself by updating Plesk as outlined in the KB you linked to, and resetting ALL your FTP, email, admin and control panel passwords.
In the unlikely event that you have a version of Plesk earlier than 8.6 then you will need to manually copy a file to a certain location to deal with the vulnerability -- using the Plesk updater doesn't help as it doesn't have the smarts, pre-8.6, to do the necessary things.
All this is on the assumption that the bad guys did not access your server and manually then access Moodle data in one way or another. This is unlikely to have happened, IMHO, but not impossible. Basically you'll need to keep a close eye on things, maybe get a pro to check things out for signs of tampering or malware, and take it from there.