According to Mochizuki himself, the essential prerequisites for the IUTeich papers are:
Semi-graphs of Anabelioids (sections 1 to 6)
The Geometry of Frobenioids I: The General Theory (complete)
The Geometry of Frobenioids II: Poly-Frobenioids (sections 1 to 3)
The Etale Theta Function and its Frobenioid-theoretic Manifestations (complete)
Topics in Absolute Anabelian Geometry I: Generalities (sections 1 and 4)
Topics in Absolute Anabelian Geometry II: Decomposition Groups and Endomorphisms (section 3)
Topics in Absolute Anabelian Geometry III: Global Reconstruction Algorithms (sections 1 to 5)
While other sources also recommend:
The Hodge-Arakelov Theory of Elliptic Curves: Global Discretization of Local Hodge Theories
The Galois-Theoretic Kodaira-Spencer Morphism of an Elliptic Curve
Particularly interesting is Fesenko's recent extended remarks on IUT (and learning IUT):
- Ivan Fesenko, Arithmetic deformation theory via arithmetic fundamental groups and nonarchimedean theta functions
There's also an introductory paper by Yuichiro Hoshi, but at least for the moment it is avaible in japanese only
- Yuichiro Hoshi, Introduction to inter-universal Teichmüller theory
As for the (considerable) gap between Hartshorne and Mochizuki's work, the references on each paper are quite concrete and helpful (see for example the ones on Topics in Absolute Anabelian Geometry I for a good sample).