If the behavior/symptoms started shortly after eating the slug, as opposed to before the slug, the slug could be stuck. The slime they produce can be like glue! I used to have a pet chicken that loved slugs, but after eating a few her beak would get glued shut. I tried lots of things to scrape the slime off her beak, but then remembered that pineapple has lots of enzymes in it that could dissolve the proteins in the slime. It worked great. So we later developed a routine -- she would go out and eat slugs until she couldn't open her mouth anymore, then she would come find me and squawk through a closed beak. I would open a can of pineapple chunks in its own juice, and she would drink the juice until the slime broke apart and I got pineapple treats. Worked out great for both of us. And after a few years I had almost no slugs on the farm! Sadly, Sydney has been gone for almost 9 years now, and none of my 35 current chickens/turkeys like slugs enough to control the slimy pests.
Other problems that can cause gaping (lifting the head into the air, stretching out the neck, opening mouth without vocalizing, +/- adjusting the crop), would be temporary overconsumption (especially of dry food), crop blockage for any reason, sour crop, and gapeworm, plus probably several other issues. If it continues for a long time with any frequency it may be a problem, but if it lasts less than a day and goes away on its own than it's nothing to worry about, as long as the chick is doing well otherwise.
