Hope your yellow watchman is okay. We have both a yellow watchman and also a mandarin. I don't see any chance that the mandarin is killing it for the cave; they are generally considered one of the most peaceful species around.

About two months ago, our yellow watchman completely spazzed out. He has always stayed in his little cave and comes out a few times a day, definitely comes out to feed every evening and occasionally to run someone off that is getting too close to the cave (including our bigger mandarin, by the way). But on this day, he started swimming all around the tank out in the open, which he had NEVER done, and was just almost spasming with his very excited swimming pace. We were almost certain that he was a goner because he was acting so strange. This lasted for almost 24 hours when he finally, very disoriented, found his way back to his cave. He has been living there peacefully ever since, eating well and seeming happily grumpy as usual. All good!

Hope this is the case for yours!