Final answer:
Macbeth's reference to 'the worm that’s fled' indicates his concern that Fleance, who has escaped, may grow to become a threat to his kingship in time, despite being harmless at present.
Step-by-step explanation:
When Macbeth refers to "the worm that’s fled", he is speaking about Fleance, Banquo's son, who has escaped after Macbeth's hired murderers killed Banquo. In the line "Hath nature that in time will venom breed", Macbeth fears that Fleance may become a threat to his kingship in the future, similar to a seemingly harmless worm that eventually develops into a venomous creature. Macbeth acknowledges that Fleance does not have the power to harm him now (“No teeth for the present”), but he worries about the potential danger he might pose once he grows up, considering the witches' prophecy that Banquo's descendants will become kings.