There are two types of Hydnophora I am aware of. A bright neon green and this less common blue variety. These Sky Blue Hydnophora frags are 3/4" to 1" in length and grow quickly.
The Sky Blue Hydnophora exhibits a very crisp color and is accentuated by delicate purple polyps extending along its ridges. This WYSIWYG frag is about 1" in length and grows quickly.