Take a look at the sentence again:
Eli held the phone on his lap, trying to work up the courage to call the
beautiful Leodine. All the while, his foot jiggled,
his nostrils flared, and
sweat was collecting on his upper lip.
When you list grammatical items, you must use parallel structure. Here, his foot jiggled and his nostrils flared are two short main clauses with simple past tense verbs, but sweat was collecting on his upper lip does not have the same form, wrecking the parallelism.
To fix the sentence, change sweat was collecting on his upper lip to his upper lip sweated.