Very common parasitic isopod that feeds off their gills. Doesn't seem to harm them, maybe slows their growth overall. Actually molts and grows with the shrimp.
I've removed these before; shrimp heads can be lifted lightly, like a flap on the sides. I use a pair of tweezers gently open their head and remove them, then hold the shrimp under the water to let any air out of the bubble-shaped carapace before closing the head again.