If you got it last week, i presume it is at least 4 days already? Were the zoa polyps open when u bought it at the farm? Also when u say it didn't seem to open up, does it mean all the polyps are closed or some of the polyps seems to open a bit?
If the zoa polyps were open when u bought it at the farm, you could take it out of your tank and dip it in a bucket of tap water. Shake it in the water for 30s at most and you will see a lot of things falling out of the rock.
After that put the zoa rock back into your tank and wait to see if it opens up in the next 2 days.
If the zoa polyps were not open when u bought it at the farm, then take the zoa rock out of the tank and smell it. If it stinks, most probably you will have to throw it away.
The main reasons why zoas do not open are:
1) Stress due to transportation from where it is harvested to Singapore
2) Die offs on the zoa rock which fouls up the rock
3) Nudi/pest attack on the zoa rock
4) Zoa pox