I bought a frag o' xenia about 2 weeks ago and it's really doing well. But I just noticed last night that between some of the xenia stalks and around the fringes of the "rock"(which is fingery), there is some yellow stuff, which I think is actually remaining stony coral.
I can't see any polyps clearly, but there are some tiny uniform bumps on it, I was thinking maybe yellow Porites.
It may look that way from the pictures, but this is some SPS for sure. Sponges don't need light right? This things growth line ends exactly where the xenia shades it. I felt it last night. Very hard. Also, the tissue matches the skeleton it's growing on. There are tiny tiny bumps (sorry, can't photograph well) that I assume are either really tiny or retracted polyps - their spacing matches the dead coral skeleton it's growing on. Basically, some of it survived. I am now thinking porites cylindrica.
Third picture shows bumps the best. I'm guessing I won't get a positive ID without at least a better camera. I'd like to know what it is so I can afford it proper care. Unfortunately, it seems the reported optimum conditions for porites and xenia are verrrry different.
You can cut them off the xenia colony, and just start a new frag growing. I have done that with hitchhikers in the past. That way, they can each get their proper lighting requirements. Always fun to find these surprises!