I wonder if she drools a lot. Her mouth is usually hanging open, and it's not uncommon for people with severe neurological disabilities to
struggle with managing oral and respiratory secretions because of a diminished ability to swallow and cough. The skin's being constantly wet can cause redness or irritation, and saliva can be particularly irritating because contains enzymes that help break down food. Even healthy people who happen to be especially drooly sleepers may sometimes get a rash around the mouth where it touches the pillow.
We have seen Luna's "swallow" during her
waterboarding sessions feeding therapy. She doesn't have a safe swallow and I wouldn't be surprised if she drools most of the time. We have also seen some indications that she has very sensitive skin, which is common enough among babies and young kids.
Incidentally, there are ways to manage chronic drooling for disabled people. Pooling of saliva and secretions in the mouth and throat poses a major risk for aspiration, which can lead to potentially fatal pneumonia. It's also a matter of personal dignity - caregivers sometimes pursue treatment for the excessive drooling so that an older child, teenager, or adult doesn't have to wear a bib or have someone wipe his mouth for him, even if the person doesn't appear to be aware or bothered by it. Excessive drooling can also ruin clothing or damage electronics, which is especially relevant for disabled people who use assistive technology.
Treatment options include surgical removal of some of the salivary glands, Botox injections, oral medication, or a medication delivered transdermally (through the skin) via a small adhesive patch worn behind the ear.
Robyn has proven not to care about Luna's comfort or dignity, though, so I guess she'll just keep drooling all over.