Games like Mortal Decay
Games like Mortal Decay include Outlast, Outlast 2, Resident Evil 7 Biohazard, SOMA, and Poppy Playtime. We matched them by the tags players give Mortal Decay on Steam (Horror, First-Person, Survival Horror, Psychological Horror, and Atmospheric): closest matches first, and among equally close ones the better-known games.

Outlast
Shared: Horror · First-Person · Survival Horror

Outlast 2
Shared: Horror · Survival Horror · First-Person

Resident Evil 7 Biohazard
Shared: Horror · First-Person · Survival Horror

SOMA
Shared: Horror · Psychological Horror · Atmospheric

Poppy Playtime
Shared: Horror · First-Person · Singleplayer

Resident Evil Village
Shared: Horror · Survival Horror · First-Person

METRO 2033 Redux
Shared: Horror · Atmospheric · First-Person

Metro Exodus
Shared: Singleplayer · Horror · Atmospheric

Amnesia: The Dark Descent
Shared: Horror · Survival Horror · Psychological Horror

The Outlast Trials
Shared: Horror · Survival Horror · First-Person

METRO Last Light Redux
Shared: Horror · Atmospheric · First-Person

Five Nights at Freddy's
Shared: Horror · Survival Horror · Singleplayer

Five Nights at Freddy's: Security Breach
Shared: Horror · Survival Horror · First-Person

The Forest
Shared: Horror · Survival · Survival Horror

Dead by Daylight
Shared: Horror · Survival Horror · Survival

The Beast Inside
Shared: Horror · First-Person · Survival Horror

Darkwood
Shared: Horror · Survival Horror · Psychological Horror

Five Nights at Freddy's 2
Shared: Horror · Survival Horror · Singleplayer

Alien: Isolation
Shared: Horror · Survival Horror · First-Person

Bendy and the Dark Revival
Shared: Horror · First-Person · Survival Horror

Amnesia: The Bunker
Shared: Horror · Survival Horror · Atmospheric

Slender: The Arrival
Shared: Horror · First-Person · Survival Horror

Escape the Backrooms
Shared: Horror · Psychological Horror · Survival Horror

The Evil Within 2
Shared: Horror · Survival Horror · Psychological Horror