Games like The Moon Sliver
Games like The Moon Sliver include What Remains of Edith Finch, Firewatch, Layers of Fear (2016), SOMA, and Amnesia: A Machine For Pigs. We matched them by the tags players give The Moon Sliver on Steam (Walking Simulator, Psychological Horror, Adventure, First-Person, and Exploration): closest matches first, and among equally close ones the better-known games.

What Remains of Edith Finch
Shared: Walking Simulator · Story Rich · First-Person

Firewatch
Shared: Adventure · Walking Simulator · First-Person

Layers of Fear (2016)
Shared: Psychological Horror · Walking Simulator · First-Person

SOMA
Shared: Psychological Horror · Horror · Story Rich

Amnesia: A Machine For Pigs
Shared: Walking Simulator · Psychological Horror · First-Person

The Beginner's Guide
Shared: Walking Simulator · Story Rich · First-Person

Outer Wilds
Shared: Exploration · Adventure · Story Rich

Gone Home
Shared: Walking Simulator · Exploration · Indie

Still Wakes the Deep
Shared: Psychological Horror · Walking Simulator · First-Person

MiSide
Shared: Psychological Horror · First-Person · Story Rich

Little Nightmares
Shared: Adventure · Horror · Atmospheric

Mouthwashing
Shared: Psychological Horror · Story Rich · Horror

The Park®
Shared: Psychological Horror · Walking Simulator · First-Person

Little Nightmares II
Shared: Adventure · Psychological Horror · Horror

INFRA
Shared: Walking Simulator · Exploration · First-Person

Blair Witch
Shared: Psychological Horror · Adventure · First-Person

Sally Face - Episode One
Shared: Psychological Horror · Story Rich · Indie

Among the Sleep - Enhanced Edition
Shared: Adventure · Indie · Walking Simulator

Outlast
Shared: First-Person · Psychological Horror · Horror

Poppy Playtime
Shared: First-Person · Horror · Psychological Horror

Scanner Sombre
Shared: Walking Simulator · Exploration · Adventure

Fran Bow
Shared: Psychological Horror · Adventure · Story Rich

Layers of Fear 2 (2019)
Shared: Psychological Horror · Walking Simulator · Indie