That looks like fire damage on the sheets.
If you recall, cinnamon-bun mom said his father could breath fire.
Given Deku's personality its possible he's a late bloomer for 'batin.
Considering the above… Deku's secret quark is to cum fire!
Don't go Uraraka! You can't possibly survive the Deku smash!

You brave foolish girl.

Ochako: What's going on, here?! I just heard a massive explosion!!
Aoyoma: Well, it appears that young Izuku Midoriya has just Florida Smashed his bedroom in his sleep.
Ochako:…..Tsuyu, tag in. You're my friend, so you get him.
Tsuyu: Todoroki, tag in. I can't do hero work if the lower half of my body is broken.
Todoroki: Bakugou-
Bakugou: Death-glares
Todoroki: Momo tag in.
Momo: I don't even like him!!
Mina: I'LL TAG IN!! :D
Ochako: NO THE HELL YOU WILL NOT!!
Midoriya: What are you guys talking about?!! o___o
Aoyama: Not until you are older, sweet summer child. :)