Yes they will eat coraline algea my rock was covered in coraline and now there are spots where you can tell where the asterina sit and the coraline is gone
There is no end to the Asterina discussion here. Its split between those that claim the eat Zoa's and other corals, and those that claim the are harmless. I'm in the 2nd camp. Not only are my Zoa's untouched, but they also do a good job of cleaning the glass. I haven't seen them touch coralline.