In D&D5 everything with a plus/minus stacks with every other thing with a plus/minus.
Things that set a value do not stack with other things that set a value but do stack with things that give a plus. For example, the Mage Armor spell sets your AC to 13 + DEX bonus; it does not stack with a draconic sorcerer's natural armor which also sets your AC to 13 + DEX bonus, however, either will stack with the Shield spell which adds +5 to your AC.
The spell doesn't specify that a hostile creature must stop in the space, only that if they move to (meaning moving to or through, because moving through a space requires first moving to it) a space within 10 feet, it is affected.
This can be simplified to
Any creature moving within 10 feet of the guardian must succeed on a
Dexterity saving throw. The creature takes 20 radiant damage on a
failed save, or half as much damage on a successful one.
This would be useful in many situations where you wouldn't want the enemy to advance on the party, and certainly much more useful than if it only applied to a creature that stops within 10'.