It would not seem that it is an environmental issue because I would think they both would react negatively, not just one of them. It seems more localize problem, perhaps it has been picked on or irritated. It is possible that mucus containing sting cells from another coral has contacted it (toad stool are bad about doing this) Might be preparing to split / divide. The transparency could be zooanthely bail out or just now that the tentacles are thicker, the algae is not as dense. The description unfortunately is not indicative of a specific ailment.