It's been pouring with rain for the passes 3 days and after the first day I moved a lot of plants under a carport and my Medusa's head has ended up covered in this mould.
It doesn't seem to be affecting anything else.
Under the carport has good airflow too

You're right, its mold. There may be good air flow around the plant but the is no air flow through the plant (the attack of the prepositions). I would figure out how to get rid of the mold. Mold growing on plants is never a good thing. No, no clue.

Thank you Daisy
I've sprayed it down twice with a fungicide and it has been left out in the open, today is meant to be nice and sunny late 20s so hopefully that stunts it.
If it doesn't fix it then I'll pull it out, cut off the affected leaves and treat it with maybe a weak hydrogen peroxide or something

Sounds drastic! It seems warmer weather and a breeze would solve it but I've never had actual fuzz growing on a succulent (on my dishes a couple times ). Skip the hydrogen peroxide.

I don't know about treatment, but I have some good news (sort of) here. This is a tangential aside, not directly relating to Jim's medusa, but still relevant.

There is a type of fungal pathogen (mildew) which specializes in Euphorbias. I had it. It was horrible, persistent and really hard to get rid of. You do not have it.

More here:

https://www.euphorbia-internat...

The powdery mildew that grows only on Euphorbias is really hard to treat. I tried, and I talked to a long-time nursery owner who did too. She ended up taking all the plants out of her greenhouse and spraying down all the surfaces with some serious spore-killing chemicals before it could be inhabited by Euphorbias again.

The problem is the spores go everywhere and sit there waiting months or years to activate, so even long after you've trashed every Euphorbia that was affected, others continue to get it. It's been a few years now since I saw it but I'm still alert for the slightest sign. There are a couple of good pictures on that pdf if you want to know what to look for.

Hopefully it does, I only suggested hydrogen peroxide because it was used in a video I watched on soil surface mould. After reading through Bajas link, I might be able to use lose dose bicarb soda or something I also bought copper sulfate to make a bourdex mix (or however it's spelt).

The plant has been outside in the sun all day and looks a little better but unfortunately it's losing a lot of arms to this mould, it seems to suck the life out of it.
I might repot it into a smaller pot that give more air flow to the arms.

So I cut off the arms that had mould on them, it ended up being about ~10-15 of them and the ones that had it were all dead inside, no sap came out when cut and they were sapless even into the healthy part of the arm (if there was any)
I've also repotted it in a pot that's about the same size at the root ball but which gives the arms less dirt to touch and more of an over hang.
The arms being cut off has also opened it up a bit.

It sounds like it was good you decided to be pro-active about the problem.

I have a not so good update on my Medusa's head. It was perfectly fine and looking really good on Monday arvo (currently Wednesday night in the land of Aus) and I just went outside to check something and found it like this. It had 2 cockroaches crawling around the side of the arms, could they have done this?

That's pretty serious damage, I would think something bigger and hungrier than cockroaches caused it.