Why does my GUI look like this?

I don’t know what the problem is. Whenever I open Mocha HitFilm, it looks like this

Are you talking about the interface or the viewer?

The footage looks like you’ve got an alpha key or similar from Hitfilm and it’s being multiplied. Have you done a key on the footage back in HitFilm?

On this specific footage, there was a key. But this is what all my footage looks like in the viewer, with or without keying and masks

This looks to me like there is alpha coming through. If you can send an example clip I can check what’s happening, but this definitely just looks like your layer has transparency somewhere.

What specific version of HitFilm is this?

I have hitfilm pro 2021.1
I removed all transparency other than the mask when I went into mocha. But even with raw footage that doesn’t have any keying, masking, or transparency it still looks like that

What is the footage format and codec? ProRes, TIF, HEVC, H.264 etc? What was it shot with?
Can you show me what it looks like in the HitFilm view?

The format is 16-bit mono. The codec is H.264
I shot it on my iPhone X

I just applied mocha directly to the footage

Okay, that’s weird. Can you DM me the footage or email to martinb [at] borisfx [dotcom]? I’d like to see what’s going on here.

So far I can’t see why that would be happening from the clip you sent. Try a fresh comp without any of the effects applied (even turned off) and see if it behaves.
Failing that, go to Hitfilm options and delete the caches, then try again.
I only see this view in Mocha if the key and mask has already been applied in HitFilm.