"He wanders away from the group, muttering something about fingers and toes." - The hunger game, Mockingjay.
And can you give me the name of this grammar structure? Is it short form of relative clause?
In this sentence, muttering is a present participle (verb form) in a phrase that serves as an adjective modifying He. It could have been structured
He, muttering something about fingers and toes, wanders away from the group.
Muttering takes an object, something, which is further modified by the adjectival prepositional phrase about fingers and toes.
The verb form mutters is the third person present tense form, and would only be used in a clause, which would require a subject. It could have been used as follows
He wanders away from the group, [while he] mutters something about fingers and toes.
With the conjunction (while) and subject (he) missing, the sentence has a bit of a run-on quality, which is not unusual in literature. In fact, including the full structure would sound a bit stilted.