Anemone with gaping mouth

[sp_dfw]

Hello. I'm about 6 months into my current 5g reef tank, but this is my first time with anemones. Over the past couple of days my larger green BTA has been withdrawing with a gaping mouth. The clown it's bonded too is desperately trying to keep it happy. My water parameters seem to be fine. I have a slightly elevated magnesium count, but not super high. No ammonia, no nitrate/nitrate/phosphate etc. Calcium and alkalinity are in range. pH runs a little on the low side, 7.8 to 8.0. The smaller green nem seems to be fairing better. How concerned should I be?

 

[Gumbies R Us]

@OrionN