Thoroughly cleaning the entire seatpost head assembly would be prudent. I would stay away from any grease or paste there, unless the problem returns (after being solved for a period of time).
Remove the post, remove the seatpost clamp. Clean it all, especially the split in the seat tube and deep into the tube. Put a smear of grease between the clamp and frame (regardless of material). DO NOT use assembly paste there. DO use assembly paste between the frame and post, but only apply it to the post at about the clamping area to 1inch down.
Creaks can come from anywhere. Sometimes the post is allowed to rock fore/aft in the frame due to insufficient clamping force. Depending on clamp orientation, friction between the clamp and frame can hinder tightening. Other times, the post and frame fit is a little loose, and even when it's clamped well, the lower part of the post will rub the inside walls of the seat tube when the frame flexes. Assembly paste, while great for clamping, can lead to creaks when not being fully clamped, like down low in the seat tube.