Not so long ago I got my hands on a box full of loose bricks. There I found an old minifigure head of Darth Maul BUT stuck in his hood! I'm 100% sure it's glue. Is there anything I can do about it ?
To paraphrase an answer from @awe on a similar question,
You can use some sugar liquid as temporary glue to make the head stuck to the torso. Wait until the sugar is sticky and then pull off the helmet. Once the helmet is loose, put the head and torso in warm water to remove the sugar glue.
Depending on the glue attaching the hood to the head, you might need to use some other water-soluble glue in place of sugar water glue.