Two things possible with this toadstool leather,
1) damage from pest (worm like a bobitt)
2) bacterial infection
If pest- Lights out, red light search for pest. Remove pest.
If Bacterial- treatment for it can be done with erythromycin. This would require removing the leather for treatment. Second option would be to cut above the damaged area and plant it elsewhere to reattach, thus cutting away the infection. Scrape away the infected tissue from the original location.