When Grendel says Unferth's "beard and hair straight down like seaweed" (86), what literary device is being used?
Saying that something is like something else is simile.
Example: "His hair was like string."
Saying that something is something else is metaphor.
"His hair was yellow yarn lying about his face."
The two devices are achieving the same effect, i.e., conjuring up an image in the reader's mind by comparing one thing to another. However, metaphor has a stronger effect than simile.