That's what's referred to as "Chalky" halibut.
As described from ADF&G publication:
What is “chalky” halibut?
It is not a health hazard to humans. The flesh of
chalky halibut is bright opaque white, rather than
translucent like normal flesh. When cooked, chalky
halibut is drier, but has acceptable flavor and higher
oil and protein content.
Chalkiness is not necessarily visible at the time
of landing but may develop over a period of hours.
It is caused by a buildup of lactic acid, which reduces
the ability of flesh to retain water. Important
factors include exhaustion, water or air tem
perature, and handling. Chalkiness is also more
prevalent in August. Overall occurrence in Alaska
is about 5%.