Yes, a later seal. I'd say first half 13th century. These metric legends are always hard to read, especially with a fragmented
legend. I agree with your readings of the names Alexios and
Johannes, the last of which is beyond doubt. Sebastokrator is also very probable, with the clear A-ligate CT in the first line and then KPATOP.
So we have two first names, and Ioannou being the
genitive, this is likely a
patronym. I think the
obv. reads ...AΛEΞION H CΦPAΓIC ΓPAΦEI ('the seal writes Alexios...
It's not yet clear to me if Alexios is sebastokrator himself or the son of a sebastokrator named
Johannes.
Regards
Gert