Last weekend I noticed several of the tentacles on my torch were completely white. What once was tan/maroon in color was now absent of any color. But it looks like just one head affected. My par.-
PH - 7.94
Salt 36
Temp. 80.1
Alk. 9.3
Calc. 479
Mag. 1360
Phos. 0.09
How long have you had this torch?
From the appearance, I think it's an Australian torch coral, which is very picky in terms of water parameters compared to the Indonesian counterpart.
It's possible that it is upset from being introducing to a new environment or from a parameter swing.
I would recommend checking for nitrate and whether there are any significant swing in water parameters.