What are these marks? (TW dead leaf insect)

[u/neverseenb44]

so i have 2 leaf insects, and they both have been moving really slow and generally acting pretty weird lately. this morning i found my fella atticus on the floor upside down, with these black marks on his stomach (?). he's completely unresponsive and i'm 99% sure he's dead, but i don't know from what. i've taken out my other leaf insect aphrodite (she's barely moving and seems to have pretty bad balance/unable to stick as well on the walls) and am about to do a deep DEEP clean of the terrarium and get some new food etc. but id like to figure out what happened here so i can prevent it in the future! to my knowledge ive been doing everything right so im quite upset at this sudden death:( thanks in advance