  In addition to occupying the extreme, diffuse tail of the dwarf galaxy
population, Ultra-Diffuse Galaxies (UDGs) are themselves a key laboratory in
which to study star formation in extreme low-density environments. In the
second paper of this series, we compare the spatially resolved star formation
activity of 22 HI-selected UDGs and 21 "normal" dwarf galaxies within 120 Mpc
to predictions within the pressure-regulated, feedback-modulated (PRFM) theory
of star formation. To do so, we employ a joint SED fitting method that allows
us to estimate star formation rate and stellar mass surface density from
UV-optical imaging. We find that the PRFM framework extends successfully to the
UDG regime - although the UDGs in our sample show unusually low star formation
rate surface densities given their HI content, this low star formation
efficiency can be naturally explained by the diffuse structure of the UDGs. In
fact, when cast in the PRFM framework, the relationship between midplane
pressure and star formation in the UDG sample is in good agreement not only
with the "normal" dwarf reference sample, but also with measurements from more
massive galaxies. Our results suggest that despite their low star formation
efficiencies, the HI-rich UDGs need not be forming stars in an exotic manner.
We also find that the UDGs are likely H$_2$-poor compared even to the overall
dwarf population.
