UPDATE: Crisis Averted.
So...it's both not what I thought it was, and what I thought it was.
Turns out my 5 year old phone has developed some rendering issues. The message I can only assume is the result of an image being turned sideways getting caught in the renderer's cache or buffer and presenting as a 'ghost' similar to burn in. The image in question was a meme right here on lemmy:
https://lemmy.zip/post/4349105
And presented as below but with less than 1% opacity.
====== Original.
I found a device level messaging overlay on my Huawei device that sits above the application layer. It displays a very low contrast transparency with more transparent text. The imagery is not burn in, I thought it was which is how I found it. Moving a mottled grey image around on the screen in a low light environment makes the text more visible. The text in question seems to be an anti-porn message reading along the lines of "THE NUDITY IS FUN UNTIL YOU WAKE UP AND REALISE YOU'RE OVERWEIGHT"
The image is not permanent and is not always visible.
This messaging would seem in-line with Huawei's own anti-porn filtering service. It seems that Huawei have their anti-porn filtering service running for non-Chinese devices sold outside of China. It would seem they've tried to hide the anti-porn system rather than disable it on foreign devices.
I'd be interested to know if anyone else with a Huawei device has noticed anything similar or is able to confirm that the same occurs on their device. For reference mine is an Australian bought Huawei nova 3i model INE-LX2.