It kind of looks like that but only appears to have spikes on either side of it body. It also on comes out at night time and looked to be over a foot long and as big around as my finger when I first saw it. I will try and get a better pic.
If it is a cucumber do they hang out in rocks and only come out at night. This one comes out of the same rock the last 4 nights I had never seen it before then. I thought that cucumber were more of a sand dwelling creature. Not to say you guys are wrong because I am stumped just asking.