The "tree" is the skeleton. It just sounds like it's retracted more than normal. If everything else is fine, it might just be in a "bad mood". My LPS
corals go through phases, maybe every 4-5 months or so, where they won't expand fully like normal. That phase will last a couple days, and then it will be back to normal.
Only think I've been able to figure out is that maybe they're saving the energy normally used to expand, and using it to go through a growth spurt - much like a child will sleep/nap more during a growth spurt. Just my unscientific thoughts, though.