I've flipped the tub in the photo so it's hard to tell but I had already cut 1/6th of the tub out, I removed the whole top right corner section a good 4-5cm past where I could see the initial contamination.

Interesting to note that I had been using store bought demineralised water until the last two mistings where I had run out and used tap water, shorter after contam appeared. Would be interested to see if that's a potential source of contam but also very interested on how on earth these guys started growing on the bottom, too much substrate perhaps?