The text does not necessarily require that John is speaking to an angel or to one of the angels previously mentioned. Although many versions insert "the angel said" in verse 9 (e.g. ESV, RSV), the Greek text merely says "he said" (actually "he says" - present tense; λέγει). No manuscript variant contains the word "angel" (ἄγγελος). Andrew of Caesarea (563-637), who wrote the first complete commentary on Revelation, however, inserts "the angel" when quoting the verse and exposits v.10 with the understanding that it is an angel who is speaking.1
Orthodox commentator Lawrence Farley offers an explanation that the "he" in verse 9 was actually the voice from the throne recorded in v.5. John, supposes Farley, mistakes the voice for Christ Himself, whereas it is actually a special angel sent by Him. He explains:
I would suggest that Christ’s angel sent by Him with His revelation for John (1: 1) resembled Christ Himself. Certainly in the case of guardian angels generally, it would seem they bear an outer resemblance to those whom they guard. Thus, when St. Peter was released from prison and stood at the door of Mary’s house in Jerusalem seeking entrance, some thought it was not Peter himself but “his angel” (Acts 12: 15). Apparently one’s “angel,” then, looks rather like oneself.
Not, of course, that Christ our God has a guardian angel as we do. Rather, this angel would be a special servant, sent to carry His message and His Presence, and as such looked like Christ, whose Presence he carried.2
Farley goes on to surmise that John inserts this episode of mistaken identity - along with that in 22:8-9 - in order to address the contemporary and heretical practice of angel worship that was developing:
Angel-worship was indeed a problem to the early Church, especially in Asia Minor. In Isaiah 9: 6 (LXX), the Messiah is referred to as “the Angel of Great Counsel”— i.e. as the Messenger (Gr. angelos) who carried out the divine will. Such angelic titles were used for Christ in certain centers of Jewish Christianity.
This need not have been a problem, but some drew unwarranted conclusions from it. For some groups, with gnostic tendencies, suggested that the Messiah was, in fact, an angel— that is, a created being. Exalted above all others, to be sure, but a created being nonetheless. This was heretical and not a valid conclusion from the (admittedly ambiguous) Jewish title for the Messiah in Isaiah 9: 6. It was in flagrant contradiction to the apostolic deposit of the Faith, which asserted in no uncertain terms the full deity of Jesus Christ.
Nonetheless, this teaching continued to spread in Asia Minor. The Epistle to the Hebrews rebukes the idea that Christ is an angel (see Heb. 1: 5f, “To which of the angels did He ever say, ‘You are My Son’?”). St. Paul rebukes the teaching in his Epistle to the Colossians (see Col. 2: 18, “Let no one disqualify you by delighting in the worship of angels”).
Because of the prevalence of this heretical practice, John records his own mistake as a kind of enacted rebuke of the heresy. None in the churches may follow the heretics in the adoration of angels; such worship is for God alone. One may not give the worship due the Father and the Son (such as is given in 5: 13) to created angels. The Church must take care not to reduce Christ to the status of an angel. For the witness of Jesus is the spirit and essence of all true prophecy.3
1. E. Constantinou, Andrew of Caesarea and the Apocalypse in the Ancient Church of the East: Studies and Translation (Ph. D. Thesis, Université Laval, 2008)
2. The Apocalypse of Saint John: A Revelation of Love and Power