Grey patches in sand

[Errenmay]

Hi everyone, I've had this 10g tank for 5 weeks, lots of plants and a fish-in cycle (almost done). I noticed these grey patches appear and I am scared if I disrupt them it will leech toxic chemicals into the tank. We removed a cichlid cave and it smelled extremely horrible but our fish were totally fine, I'm not sure if it's related. I also wanted to remove some of the sand if possible.

 

[Autumnsky]

The areas are stagnant, and anaerobic bacteria grow, you can get pockets which can make hydrogen sulfide, can smell like rotten eggs / more info in linked thread below.


https://www.aquariumadvice.com/forums/f12/hydrogen-sulfide-the-answer-you-asked-for-147656.html