Grand Duke Alexey Alexandrovich
The man doesn't resemble GD Alexei at all. That's not him.
Did you post a real picture of Alexei to prove that naval man is him?
In the 1st "GD Alexei" picture is not Alexei. My argument: the photo of that naval commander was taken in the 1870s - those years such beard-style was very fashionable, and GD Alexei also worn such a beard, but in the 1870s he was a young man, not that middle-aged as in the photo. Later, when he was older, he worn the style of the beard one can see in the real photo of him.
And, by the way, the unknown man has the straps of an Admiral, and GD Alexeli was a General-Admiral.