The Plateau of Leng (a CCC card) says "Action: Exhaust to choose a character committed to a story. At the end of the phase, that character loses all of its printed [terror] icons and goes insane, if able".

Does the target character HAVE to have printed [terror] icons to be eligible to attacked with this power? And for that matter, say a character with no printed [terror] icons has an attachment that grants it [terror] icons, does this card allow you to disregard those?

Yes, you can target a character with no [TERROR] icons... in FAQ 2.0 you can read:

(v1.8) Icon Removal

You can “ remove ” an icon from a character who
does not have that icon, but the modified number
of icons that character possesses will still be 0.

(The last sentence only indicates you that never would be negative numbers of icons.)

But you cannot remove icons not printed in the character with PoL, like thoose granted by Support or Event cards or by triggered Actions or Responses.