Quote from another site.
25 Reasons why your polyps won't expand.
1. You have a zoanthid eating worm.
2. Sundial snails. ( I've experienced this )
3. A fish that is nipping at them, there are several well documented cases of this. ( I've experienced this )
4. An invert doing the same. ( I've experienced this )
5. Unstable or fluctuating parameters.
6. Large Emerald Crabs or a Sally Lightfoot. Both can and will
consume, kill and cause polyp retraction. ( I've experienced this )
7. Sea Spiders. ( I've experienced this )
8. If shipped, the water was much too cold and you placed it directly into your tank without slowly drip acclimating them. Or, the inverse, the water was too warm.
9. Nudibranchs ( I've experienced this )
10. Stray Voltage. ( I've experienced this )
11. Excessively high Iodine which would prove fatal.
12. Light shock
13. Polyps that were kept in PC or VHOs and then placed mid to high level under MH's without light acclimating them. Possible burning could also occur.
or
Polyps that were in maybe dual 175 SE MH and then placed mid to high under 400 watt DE MH , will cause possible burning or a delayed expansion.
14. Other stinging corals.
15. Chemical warfare.
16. Overpowering current. ( I've experienced this )
17. Excessive collection of sediment.
18. Nuisance algae around the polyps which will irritate them and prevent them from expanding. ( I've experienced this )
19. Temperamental, yes, they are, there may be nothing at all wrong, that simply don’t want to expand for that day.
20. Fungus
21. Bacterial infection. ( I've experienced this )
22. White lesions also referred to as Zoa Pox. ( I've experienced this )
23. Amphipods, which I have only witnessed eating sick, dead, dying, decaying polyps. Doesn’t mean they won’t eat perfectly healthy ones, I just haven’t witnessed it, some have.
24. Extreme hypo or hyper salinity
25. And the final reason may not be any of the reasons above. Why? Periodically, polyps will retract from days to 10 to 14 days, during which they will clean themselves externally. Then without any advanced notice, they will unfold like a flower or a summer’s morning.