if you look at the galery, there's a cg chest that said something like "the memory in the chest is covered by a sticky slime, maybe you have to experience the same" something like that
Viewing post in [RPG] GROVE: "Dark Places" (v0.308) comments
ah unfortunately there is a known bug with 0.122a that if you’re trying to unlock the slime suit chest, you can only do it before you defeat the boss. Use an old save file and get suited, remove it then you will unlock the slime suit in the gallery. Hope it helps, and apologies for the bug. It will be fixed for 0.2
-RegalBuster