Yup, it was the yellow spot, but I found another just like it locally. I have no idea what happened to him. I noticed he was not in his normal spot. Before he died, he spit out this mucous clumpy thing.
Rock is in, pics when it clears. Added some live bacteria so it may take a bit. We are trying to decide how we want to do a light fixture. May go with a wooden project box from Michael's for the base.