My dog is lip smacking, won't eat, and hiding. What should I do?

Wally is showing signs of potential distress and may be in need of medical intervention. Ideally, he should be physically examined by your veterinarian to gather information about what is happening internally with him. Lab work (CBC, chemistry panel, urinalysis, fecal exam) may be recommended after the exam to diagnose or rule out systemic infection and to assess major organ function..
The lip smacking could indicate either nausea (dogs will salivate a lot and become nauseous due to illness), severe dental disease (infected teeth are painful and can make it hard to eat) or increased stress/anxiety. Without an exam, we don't know how to best help Wally until we know what we are treating, which is why getting him seen at this time is the next best step towards his care.
Until you can get him in to the vet you can offer Wally some slightly warmed baby food (or force feed him a little) and small amounts of unflavored Pedialyte to help keep him hydrated. Hopefully, this will tide him over until he is seen by your vet.