In the Bible and Spirit of Prophecy we find no verses or statements to indicate that Melchizedek was anything other than a priest and king and we can say with certainty that Melchizedek was NOT the Holy Spirit.
A letter was reportedly written by Elder George B. Starr in which he stated that Sister White said: “I will tell you who Melchisedec was. He was the Holy Spirit, the third person of the Godhead, who took the form of humanity and represented the Lord Jesus Christ to that generation.” This report is controversial because it was made in 1924, nine years after Sister White died and several years after the meeting occurred. With regards to such reports, Sister White wrote:
“The cases mentioned will serve to show how little reliance can be placed upon reports concerning what I have done or taught. … Beware how you give credence to such reports. … And now to all who have a desire for truth I would say: Do not give credence to unauthenticated reports as to what Sister White has done or said or written. If you desire to know what the Lord has revealed through her, read her published works. Are there any points of interest concerning which she has not written, do not eagerly catch up and report rumors as to what she has said.” – 5T 694-696.