It was Ganymede according to Astronomy 2018, wanted to get some avis' of the transit but cloud and an obstructed eastern view spoiled that idea.
There will be more in the months ahead.
I use the charts Ian Musgrave posts on his web site Southern sky watch. Scroll down to see all the transit times and dates for Jupiter's satellites for the month. http://www.users.on.net/~reynella/skywatch/ssky.htm
Get the JupiterMoons app from sky and telescope; worth every cent! If you were up later you would have also seen Ganymede transiting as well as Io and it's shadow; all this between 5:30 and 6:30 this morning. Ganymede's disk was easily seen as a bright globe against the darkened limb of Jupiter and it gave a strong 3D appearance even with a mono eyepiece.
If you get the app you can tap on the disk of Jupiter to zoom in and zoom out.