OOOH your right. I've just googled water squirt images and struggled to find photos then one cropped up under the name of sea squirt, which i presume is another name for them.
I trawled through the images and found one that even shows them in the same kind of pattern as mine. I've also got other things on the back of the same piece of rock that I now know are also water squirts, but must be a different kind.
The rock I bought from one particular shop has amazing life growing on it now. It had a lot of red see weed looking stuff on and purple flower looking things on when I got it, but now theres even more unusual things appearing. I've also got a few fan worms cropping up all over the place. I just need something that loves eating green algae as these is loads of it growing.
Are these fan worms okay too? Hoping to get the rest of the rock this weekend so will be going back to the same shop the good stuff came from.