  Transiting planets in nearby young clusters offer the opportunity to study
the atmospheres and dynamics of planets during their formative years. To this
end, we focused on K2-25b -- a close-in ($P$=3.48 days), Neptune-sized
exoplanet orbiting a M4.5 dwarf in the 650Myr Hyades cluster. We combined
photometric observations of K2-25 covering a total of 44 transits and spanning
>2 yr, drawn from a mix of space-based telescopes (Spitzer Space Telescope and
K2) and ground-based facilities (Las Cumbres Observatory Global Telescope
network and MEarth). The transit photometry spanned 0.6--4.5$\mu$m, which
enabled our study of K2-25b's transmission spectrum. We combined and fit each
dataset at a common wavelength within a Markov Chain Monte Carlo framework,
yielding consistent planet parameters. The resulting transit depths ruled out a
solar-composition atmosphere for K2-25b for the range of expected planetary
masses and equilibrium temperature at a $>4\sigma$ confidence level, and are
consistent with a flat transmission spectrum. Mass constraints and transit
observations at a finer grid of wavelengths (e.g., from the Hubble Space
Telescope) are needed to make more definitive statements about the presence of
clouds or an atmosphere of high mean molecular weight. Our precise measurements
of K2-25b's transit duration also enabled new constraints on the eccentricity
of K2-25's orbit. We find K2-25b's orbit to be eccentric ($e>0.20$) for all
reasonable stellar densities and independent of the observation wavelength or
instrument. The high eccentricity is suggestive of a complex dynamical history
and motivates future searches for additional planets or stellar companions.
