Semi-slug

I think this 2cm-long blob is a semi-slug - a snail that cannot retract into the shell because it is much-reduced in size. When they are resting on plants like this they curl up into a ball. I saw this one on the underside of a fern on the Border Track in Lamington National Park. I can't find enough info to make a positive ID, but it's possibly a Brisbane Semi-slug (Fastosarion virens).