White skin flakes on black breasted leaf turtle?

My black breasted leaf turtle has dry, white skin on its head. Does anyone have an idea what that might be?

Some enclosure details: 150x60cm

Humidity: between 80 and 90%

Lightning: 2 35w uvb lamps (reptiles expert, same lamps as Lucky reptile bright sun), many hiding places

Food: nightcrawlers, larvaes, turtle sticks (fluval bug bites turtle formula)

Turtle: female, 5 years old, captive bred

Thanks!!