Is this a tanalith stain?

[Dylan T]

Hi all - bit of a puzzler was hoping someone could help with. Laid a composite deck and had some pine treated sleepers to make beds round it. Left one on the deck as didn’t complete when rainy spell hit and it was there for a couple of weeks till I got back to the job. Underneath its left a brown patch as shown in photo. At first I thought this was the new deck not fading consistently due to the sleeper blocking the sun, but the colour doesn’t match up. Does anyone know if tanalith can run and if it does how on Earth I’d get it out of decking? If pressure washed it to an inch of its life and used every cleaner I can think of!

Note that it’s the brown diagonal line isn’t the photo, the other lines and patches are some weird light thing happening from the surface water on deck post cleaning

thanks