WISE J085510.83-071442.5 (hereafter, WISE 0855-07) is the coolest Y dwarf known to date and is located at a distance of 2.31± 0.08 pc, giving it the fourth largest parallax of any known star or brown dwarf system. We report deep z-band observations of WISE 0855-07 using FORS2 on UT1/VLT. We do not detect any counterpart to WISE 0855-07 in our z-band images and estimate a brightness upper limit of AB mag > 24.8 (F_ν < 0.45 μJy) at 910 ± 65 nm with 3σ-confidence. We combine our z-band upper limit

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... with previous near- and mid-infrared photometry to place constraints on the atmospheric properties of WISE 0855-07 via comparison to models which implement water clouds in the atmospheres of T_eff < 300 K substellar objects. We find that none of the available models that implement water clouds can completely reproduce the observed SED of WISE 0855-07. Every model significantly disagrees with the (3.6 μm / 4.5 μm) flux ratio and at least one other bandpass. Since methane is predicted to be the dominant absorber at 3-4 μm, these mismatches might point to an incorrect or incomplete treatment of methane in current models. We conclude that (a) WISE0855-07 has T_eff∼ 200-250 K, (b) < 80 % of its surface is covered by clouds, and (c) deeper observations, and improved models of substellar evolution, atmospheres, clouds, and opacities will be necessary to better characterize this object.