Weird Blue Hippo Tang behavior

[Craiz]

I've got a large blue hippo tang that I've had for several years and it has been behaving very strangely the last couple of days. It has a place in the rock where it goes to "bed" every night. Now it seems to be staying there all day long. It moves around a tiny bit, but never comes out from the rock. Before starting this I didn't notice anything different about the tang. No other fish or corals are acting different.

Any ideas why the fish is behaving like this or what it means?