I have not found any undesireable hitchikers yet on mine.

Kanai rock is great to build formations with. When I said that it had no or little hitchhikers it wasn't a bad thing, as I like to only have desirable hitchhikers. I have noticed one hitchhiker as of late, it looks like a transparent nudibranch and is only about 1 cm long.

I had a lot of hitchhikers and I suppose any unidentified hitchhiker is potentially bad. I did remove two Mantis shrimps from it the day I bought it, and I continually found crabs, right up until I took some to Jon's house, where we found a small crab walking around in the bottom of the box. So in other words, my batch had a lot of hitchhiker, and therefore a certain proportion are bound to be bad. For me though, it's well worth the trouble and risk.

Has anybody found any corals in this rock

Plenty, members of the family Xeniidae, the family Fungiidae (perhaps the genus Cycloseris), the order Zoanthidea, the phylum Porifera, and animals which I believe were of the family Siderastreidae. It's hard to be any more specific.

As an aside, here's something fascinating I found on Wet Web Media. I thought of Tony/Jon's orange M. capricornis colony when I read this but I'm sure it applies to many of our corals here:

"If you are keeping live Stony Corals, boring (as in digging, not yawning) species of Sponges of the genera Cliona (pictured, Cliona delatrix, the Red Boring Sponge; and Variable Boring Sponge, Siphonodictyon coralliphagum (3’s) are definitely out. However, strangely enough, if you find the Orange Icing Sponge, Mycale laevis (pictured) growing under your plate-type corals, this is not a "bad thing". This Sponge actually protects the Stony Coral from Boring Sponge infiltration."
(http://www.wetwebmedia.com/sponges.htm)

I'm pretty sure nobody has found coral on rock for the last ten years now. Most rock harvested now is stripped of anything considered a coral, as it's worth money in it's own right. So free corals aren't likely to come on rock.
About the liveliest hitchiker you're likely to get would be a crab or mantis shrimp. Sometimes you get a polyp or three, but not much more.

:lol: What I wrote should be tempered by saying, none of them were of any significant size nor did they grow, nor were they really that exciting to look at.