The "Innsmouth look" appears in the Cthulhu Mythos story, "The Shadow Over Innsmouth", and refers to a set of physical features that result from the hybridization of humans with Deep Ones. The "look" is shared by all inhabitants of "Innsmouth" and its origins are a zealously kept secret...
lovecraft.fandom.com
The "Innsmouth Look" is basically balding, bug eyed, greasy, uncanny valley shit, sort of like this
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>The characteristic features of Innsmouth's townspeople are:
great affinity for water and swimming;
bulging, unblinking, watery eyes;
acute alopecia;
resonating, gurgling voices;
shrivelled necks with strange lines (becoming gills in the end);
wide mouths with thick lips;
flat noses;
tiny ears and;
squamous grayish skin
Most people might think these traits are some sort of congenital hereditary trait, but they are actually the symptoms of a very slow metamorphosis. As descendants of both Humans and Deep Ones, the people of Innsmouth undergo a very slow transfomation that might take most of a human lifespan to complete.