Wow, everyone has been super helpful - thanks for everyone's responses!
This is my first ever brew so I'll try my best to answer:
First off, this was from a kit about 7 days after set-up (with pre-made wort, warm water, yeast). I'm living in Mauritius so there's no access to homebrew equipment so this is a large water jug and the "ring" is just the pop cap seal from the jug, and some of the weird gunk has collected on it.
There's no time gap between photos, just one with flash and one without.
And I left the big air gap because this is in my cupboard and I read a lot of horror stories about beer bombs/overflowing.
There are no rings and after reading these comments, I'm leaning towards it being yeast or Krausen, but I'll have to wait and see (especially because we're at a consistent 30°C here which really prompts mould growth)
Thanks!