Purple and yellow growth under sand

This stuff has been growing in the capped dirt in my 20g. Any idea what it could be?

Looks like your tank contains oil reserves! Quick, it needs freedom and democracy!

I've no idea what this might be. Without meaning to be flippant, it is obviously something that was/is in the dirt. This is one of the very real dangers to using dirt. This might be (I say might) bacterial, which is highly dangerous to substrate fish. It may leech out into the upper water column too.

I tried this method a year ago on my quarantine tank, to try to grow plants in it for propagation, while having it running constantly to have it cycled and ready for newcomers or treatment of current residents. I used some organic "dirt" bought st home depot. Can't remember the name/brand. It worked ok, never had this issue you're showing, although it did trap gas there and when I decommissioned that tank a few gas bubbles came up as I removed the sand and the dirt below it.

Not really answering your question, but just wanted to share, I'm bored at work.

blue green algae and anaerobic bacteria, and maybe even some fungus. Yummy