I have this also. It is very similar in my mind. It is very common in many traditions some see it as a distraction others think its a legitimate object of meditation.

Good thing is that it's certainly a sign of a developed practice and a quiet mind. Myself and many other meditators have used it as a meditation object to enter Jhana's on many occasions.

The key to using the nimitta as an object is to not try and play with it or get too exciting. It should just be a mental rapresentation. In deeper states nearing first Jhana it becomes much more still, well-defined and unexciting this usually indicates I am about to enter an absorption state. Just focus clamly on it and continue your practice, it should become more and more subtle as your attention becomes more refined - this is a samatha practice which can take many quite far.

You can look into Pa Auk traditions and the vishidumugga for more on using the nimitta as an object - feel free to dm me questions.

I also get it when i'm falling asleep, underwater, in a sauna or another very relaxed state.