Looks like mold my friend. Tarantula poo, at least from mine, are tiny little pellets and you will see a liquid running down the cage. I forget the exact name of it all, but it’s similar to a snake I believe. If you wish to see some poo my avic just shat on the side of her enclosure, she’s very ungrateful!
The interesting thing is she went out of her web to a whole other side to poo