@exitstcgeright wanted to say hello!
The two of them had not yet been introduced. Moomin had excitedly described his best friend to Ninny, and Moominmama had told her that Snufkin was “a springtime version of Too-Ticky;” she knew the little girl trusted Too-Ticky at least, and any relation to her would be deemed as somewhat safe.
So Ninny had a solid educated guess that the elf-like creature who saw her hiding in the bushes was Snufkin. He had to be, with that hat. But she didn’t know if Snufkin had been told about her, the little invisible girl living in Moominhouse. Say he didn’t know about the ghostly figure, plagued with anxiety and wracked with fear, that wandered about with no head! The normal thing upon seeing a headless child was to freak out.
She had been hiding in the bushes because that was a normal Ninny thing to do, but now that Snufkin had spotted her, Ninny realized she must explain herself-- both for the lack of a face and for squatting in the dirt. Whatever she said, it needed to be good.
“I’m not pooping--” was the unfortunate first thing out of her mouth. “Sorry, sorry, sorry, sorry, I’m just here, don’t mind me, don’t mind me!” Ninny dropped back down to her knees, returning to her hiding spot in the bushes. That wouldn’t clear anything up, but if she were lucky, maybe he’d just think it was a trick of the light.