only visible thing on screen is round green ball

My Dad recently has converted to iMac from old Windows PC. His iMac is running latest version of High Sierra. A week ago he turned on his iMac and the only thing on the black screen was a dark green round globe with what appeared to be math hieroglyphics in white letters. Below it was a horizontal row of boxes asking him for a password. being 80+ years old, he started typing in all the passwords he had written down. This display also had infected his iPad Touch which is about a year old. In frustration, he finally clicked on the globe on his iMac display and the icon disappeared. He then checked his iPad Touch and the icon had disappeared as well. He sent me an image of his locked up iMac display. I can send the iPad picture to you if that would help identify what it is. My concern is that he might have inadvertently downloaded some sort of malware which might compromise his system.

Please let me know if the picture would help in identifying this problem. Also, any information on how to remove whatever this is would be greatly appreciated. My Dad lives close to an Apple Store on the east side of Madison, WI. should he need to bring it in for help in eliminating this which I fear might compromise his system or start a ransomware attack.

Sincerely,

Jeffri