Who Gives A Fish Forums

I'd say that's pretty normal behavior for a meat coral. They tend to expand and contract depending on their feeding schedule and environmental conditions. Since you're feeding it 3 times a week, it's possible that the coral is just adjusting to its nutrient intake.

Also, 400 watts of MH lighting ...

I've had my meat coral for a while now and it's been looking great most of the time - huge and healthy, but then out of nowhere it'll shrink up and its mouth will be slightly open. This happens about 20% of the time, and I'm wondering if this is normal.

I've been feeding it pieces of shrimp about ...