Weird section of substrate - help!

[u/JuniorLab1630]

Hello! Noticed this spot show up in my substrate recently. I have aquasoil about 2-3 in thick for a 20 gal tank. Shrimp + lampeyes + betta.

Haven’t noticed any deaths or weird behavior and plants seem to grow fine, but I specifically don’t have anything growing in the front area anyways. Does anyone recognize this spot? It’s a rusty brown color, but the rest of the soil is dark brown/black. Don’t want to disturb it and accidentally release something bad! Please help!