Maybe the following is helpful:

OR Series 1, vol 15, Part 1, pg 831

"...I would remark, colonel, that the C.S. Army has one energetic man and thorough seaman now in her service in this State; I allude to Lieutenant Harby, now stationed at Harrisburg. Should the C.S. Army accept the boat, to be used for the good of the country, I would advise that Lieutenant Harby be transferred to the steamer either as commandant or second in command, whichever position he may prefer..." written by H.S. Lubbock, Agent State of Texas and Captain of Steamer Bayou Ciy on October 15, 1862.

OR Series 1, Vol 15, Part 1, pg 974

Ltr from Maj.Gen. J.B. Magruder to Governor F.R. Lubbock
February 11, 1863

"...and to the gallant Texans, under their devoted leaders Colonels Green and Bagby and Captains Wier, Martin Snyder and Harby..." (referring to participation in the naval action within the battle to recapture Galveston)

OR Series 1, vol 15, Part 1, pg 865

Ltr to Captain Samuel Boyer DAvis, Asst Adjutant-General, Austin Tex; from X.B. Debray, Colonel Commanding Sub-Military District of Houston on November 12, 1862 regarding the establishment of obstructions and fortifications to defend littorial Texas from Federal incursions.

"...At Harrisburg there are two small forts, manned by Captain Bickley's company, of Griffin's battalion, whose men are being trained in artillery practice by Lieutenant Harby."

OR Series 1 Vol 15, Part 1, pg 213

"...In addition to these troops Lieutenant Harby, late captain in the revenue service of the United States, with a company of infantry acting as artillery, was ordered on board the Neptune..."

This may explain the confusion. Harby was a former Marine Revenue officer and did command a unit aboard the Neptune, but was not "Captain" of the Neptune. It's kind of interesting that you found a source saying he was 68 years old? Where did you run across the information?