If the slender model is selected to enter the game, and the player's arm has double skin, it will be displayed as Classic double skin

I also posted one here
Luke100000/minecraft-comes-alive#224

Custom player models can't really be supported. If that mod just changes the player between the alex/steve model, it should also change that in the correct places. https://github.com/tr7zw/3d-Skin-Layers/blob/1.18.2/Shared/src/main/java/dev/tr7zw/skinlayers/mixin/PlayerRendererMixin.java#L68 basically your model internally still reports to have wide arms, so the layers are rendered that way.

Uh... let's see what the author of MCA has to say

MCAR has solved the compatibility, so close this issue

Neat.