Siderastrea is a very common coral in the wild (so a hitcher is very plausible), but for whatever reason, it just isn't sold commercially very often.
Porites is a SPS. from the pic, I would say the corallites on your coral are relatively big... much bigger than porites porites at least, so i would rule that one out. Siderastrea is definitely a plausible guess, but in all fairness it is just an educated guess. A lot of very unrelated corals look alike and identifying them can be tricky. For example, you might never be able to tell a specimen of acan from a favid like a blasto or micromussa without close examination of the skeleton (which you might never see until it's dead). Applies here too (obviously, given the debate goin on).
as for the Xmas tree worm thing, the worms can live on a lot of different corals, but you have to keep in mind that the term "xmas tree worm" is actually a broad term that designates a whole (very misunderstood) complex of species. In the wild, it seems that they are obligated to live on some kind of live coral (or else they get eaten), but in a tank where nothing will bore into their calcium tube, they *probably* could survive anywhere from rock to dead coral. Greg, the ones you see on the rocks probably just technically aren't xmas tree worms... but hell, a lot of the feather duster worms do look a lot alike. past that you can't really generalize and say that they must live on porites and porites alone... because it just isn't true. see link below. it's a good short read and clears this issue up in my mind.
http://www.advancedaquarist.com/issu...002/toonen.htm