Oh my god!! I have the SAME problem and EVERYONE thinks I'm lying when I tell them that I am legitimately deathly afraid of starfish. What makes it worse is that because they think I'm lying, they like to test me and do stupid, stupid things to see how true it is. When they see how I react they don't even know what to do because they had no idea how bad it actually really is.
Anyway, point of this is that I've been trying to find the name for this phobia too. I really thought I was the only person on the face of the planet afraid of starfish. Ughh, I'm getting antsy just even writing this. If I find the name of it though I will definitely be sure to pass it along.