This sighting is misplaced under 'Marine Invertebrates'. It should be under 'Other invertebrates', and may be a flatworm (see: Flatworms (Platyhelminthes))
@Jennyncmg the cream coloured sponge the Tethya is on, is that part of the tethya or a different sponge? The cream coloured one seemed to be growing around/under the Tethya and wasn't present in all the spots I found Tethya.