The second yellow may have gotten bullied and it seems the injuries are pretty nasty. Try taking out the other yellow for a few days. I think the other yellow will come out of hiding.
I guess this is just a case of territorial disputes, or conspecific dispute, in this case, between the two yellows, or with the other cichlids in the tank.
Any idea what species of pleco you might have there? Because those critters are pretty territorial at times too.
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