[Noob Reefer] frag won't open

I went to the aquarium store, and asked about a piece of coral because that's what I essentially want in my tank, he set me up with something basic according to him not to hard to take care of. It was green and looked like button polyps unfortunately I did not get the name but I have a picture the ISSUE is that when I bought it it was fully open and pretty but when I put it in my tank it only opens slightly ill include a picture.

I have had my tank running for a while I got
- bi color blenny
- bw damsel
- neon blue damsel
- some sort of blue orange angel fish
- emerald crab
- Sabae purple tip anemone
- coral banded shrimp

My tank parameters
29 gal
Salinity - 1.025
Temp - 79
Calcium - 440ppm
dkH - 11
Phosphate - 0ppm
Nitrate - 20ppm

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