We demonstrate how the combination of molecular beam epitaxy (MBE) and in situ scanning tunneling microscopy (STM) can be used to study both the atomic scale growth mechanisms and the electronic properties of embedded nanostructures. Our system of choice is the rare earth monopnictides (RE-V) embedded within a III-V semiconductor matrix. This system is of great interest due to a number of exciting electrical and magnetic properties, including phonon scattering for high ZT thermoelectrics [1], sub-picosecond carrier lifetimes for terahertz devices, and strong exchange coupling between 4f, valence, and conduction band electrons near the Fermi level. To date, most work on this nanocomposite system has focused on the properties of the nanocomposite with near spherical embedded RE-V nanoparticles, e.g. ErAs or ErSb nanoparticles embedded in GaAs (001) and GaSb (001). Here we demonstrate the growth of highly anisotropic ErSb nanorods embedded in GaSb (001) during MBE growth of Er x Ga 1-x Sb by codeposition. These nanorods are continuous throughout the Er x Ga 1-x Sb layer, their axes are parallel to the [001] growth direction, and they self-assemble into ordered arrays aligned along the [-110] direction. Additionally, the resulting Er x Ga 1-x Sb nanocomposite is single crystalline with a continuous Sb-sublattice and no observable defects across the ErSb/GaSb interfaces as measured by transmission electron microscopy (TEM). Using a combination of MBE and in-situ STM, we investigate the growth mechanisms step-by-step, without removing samples from ultrahigh vacuum. We show that the growth is driven by surface diffusion and wetting characteristics that enable the surface to remain remarkably smooth during growth. Furthermore through control of surface diffusion we demonstrate morphologies ranging from pristine nanorods to branched nanotrees can be formed. We also use scanning tunneling spectroscopy (STS) to measure the local density of states of the embedded RE-V nanoparticles and nanorods. Despite the predictions of simple effective mass models [2], the ErAs and ErSb nanostructures remain semimetallic and no bandgap is opened even with reduced dimensions down to 2.3nm. We use STS to measure changes in the local density of states across the ErAs/GaAs interface and propose that the interface atomic structure results in electronic states that prevent the opening of a band gap [3]. The variety of self assembled structures and preservation of metallic behavior make RE-V/III-V a promising system for high quality epitaxial semimetal/semiconductor nanocomposite interconnects, photonic crystals, and plasmonic structures.