  Observations of rotational modulations of brown dwarfs and giant exoplanets
allow the characterization of condensate cloud properties. As of now rotational
spectral modulations have only been seen in three L-type brown dwarfs. We
report here the discovery of rotational spectral modulations in LP261-75B, an
L6-type intermediate surface gravity companion to an M4.5 star. As a part of
the Cloud Atlas Treasury program we acquired time-resolved Wide Field Camera 3
grism spectroscopy (1.1--1.69~$\mu$m) of LP261-75B. We find gray spectral
variations with the relative amplitude displaying only a weak wavelength
dependence and no evidence for lower-amplitude modulations in the 1.4~$\mu$m
water band than in the adjacent continuum. The likely rotational modulation
period is 4.78$\pm$0.95 h, although the rotational phase is not well sampled.
The minimum relative amplitude in the white light curve measured over the whole
wavelength range is 2.41$\pm$0.14\%. We report an unusual light curve with
seemingly three peaks approximately evenly distributed in rotational phase. The
spectral modulations suggests that the upper atmosphere cloud properties in
{LP261-75B} are similar to two other mid-L dwarfs of typical infrared colors,
but differ from that of the extremely red L-dwarf WISE0047.
