How to Deal With a Bag of Sticky Goo

Bart: "Someone threw a bag of poop at us. We can't get out of here" Barbara: "Can I do something really cool like just trying to look through my spells? Just alling." Bart: "I have a move that I want to pull out ASAP. Don't like being sticky, by the way," and then he rolls 20 on his dexterity saving throw. The group is trapped in an area where there are two people who appear to be firing from a watchtower. They're not quite sure what kind of humanoid they are but it's either human or Elvis.