  We present a transmission spectrum for the Neptune-size exoplanet HD 106315 c
from optical to infrared wavelengths based on transit observations from the
Hubble Space Telescope/Wide Field Camera 3, K2, and Spitzer. The spectrum shows
tentative evidence for a water absorption feature in the $1.1 - 1.7\mu$m
wavelength range with a small amplitude of 30 ppm (corresponding to just $0.8
\pm 0.04$ atmospheric scale heights). Based on an atmospheric retrieval
analysis, the presence of water vapor is tentatively favored with a Bayes
factor of 1.7 - 2.6 (depending on prior assumptions). The spectrum is most
consistent with either enhanced metallicity, high altitude condensates, or
both. Cloud-free solar composition atmospheres are ruled out at $>5\sigma$
confidence. We compare the spectrum to grids of cloudy and hazy forward models
and find that the spectrum is fit well by models with moderate cloud lofting or
haze formation efficiency, over a wide range of metallicities ($1 - 100\times$
solar). We combine the constraints on the envelope composition with an interior
structure model and estimate that the core mass fraction is $\gtrsim0.3$. With
a bulk composition reminiscent of that of Neptune and an orbital distance of
0.15 AU, HD 106315 c hints that planets may form out of broadly similar
material and arrive at vastly different orbits later in their evolution.
