I have checked these creatures out and they are tunicates due to their rapid response to touch, unlike sponges.
I may only have them for a couple of weeks or months but getting them as a hitchhiker on a piece of LR is a good size as this particular species should be quite hardly. Also where the LR is (return comp.) may have promoted it's health. These creatures demand very high quality water.
Originally this LR was in my display tank and the squirts were growing on the underside in the substrate.
Anyway, couple more have spouted over the last couple of days, will keep updated in a couple of weeks.