Answer:
The kind of literary device Huck is using in the sentence is: simile.
Step-by-step explanation:
A simile is a literary device used when we want to compare something to something else in order to imply that the two things have a similarity. It has, thus, the same purpose as a metaphor, with the difference that the simile relies on words such as "like" or "as" to make the comparison, while the metaphor does not. Huck is using a simile to compare the Spanish moss to gray beards in the way they hang down from the trees:
"trees with Spanish moss on them, hanging down from the limbs like long, gray beards"