Its bleaching, thats why the color change went from a nice orange to a florescent red, Im betting its getting close to a pinkish color. The white base is a dead give away. Don't know what caused it to bleach but if it went from on top the rock to under the rock, its hiding from something. Like what was pointed out by other posters, try feeding it. If its not sticky anymore or wont take food, the only other thing you can do is up your nitrates, get them up between 5-10. If there are any SPS in the tank they wont like this, but LPS, Zoas and softies should be happy with it. With higher NO3 the Zooxanthellae
{ algae that gives it color ) will absorb it as nourishment and hopefully be enough to let the BTA pull through. Ive kept BTAs under regular florescents using this method, plus feeding. Good luck, this may take some time.
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