  We measure starspot filling fractions for 240 stars in the Pleiades and M67
open star clusters using APOGEE high-resolution H-band spectra. For this work
we developed a modified spectroscopic pipeline which solves for starspot
filling fraction and starspot temperature contrast. We exclude binary stars,
finding that the large majority of binaries in these clusters (80%) can be
identified from Gaia DR3 and APOGEE criteria -- important for field star
applications. Our data agree well with independent activity proxies, indicating
that this technique recovers real starspot signals. In the Pleiades, filling
fractions saturate at a mean level of 0.248$\pm$0.005 for active stars with a
decline at slower rotation; we present fitting functions as a function of
Rossby number. In M67, we recover low mean filling fractions of 0.030$\pm$0.008
and 0.003$\pm$0.002 for main sequence GK stars and evolved red giants
respectively, confirming that the technique does not produce spurious spot
signals in inactive stars. Starspots also modify the derived spectroscopic
effective temperatures and convective overturn timescales. Effective
temperatures for active stars are offset from inactive ones by -109$\pm$11 K,
in agreement with the Pecaut & Mamajek empirical scale. Starspot filling
fractions at the level measured in active stars changes their inferred overturn
timescale, which biases the derived threshold for saturation. Finally, we
identify a population of stars statistically discrepant from mean
activity-Rossby relations and present evidence that these are genuine
departures from a Rossby scaling. Our technique is applicable to the full
APOGEE catalog, with broad applications to stellar, galactic, and exoplanetary
astrophysics.
