Help! What's this on mattress?

[Sleepprep5]

We have a little bed on the floor in our bedroom so DD can come in at night when scared. She's finally outgrown it and happy to stay in her room. As I was packing I away I noticed these stains all over the underside. It is inside the covering, but there is nothing on the mattress itself.

I have an absolute fear of bed bugs so it's made me feel queasy. I think it's unlikely as it is quite uniform all over and the mattress right up against it has nothing at all, it's pristine white. We've also had no bites or anything.

But some explanations of what else it could be would help put my mind at ease.

Sorry for the gross picture.

Picture now showing

Looks like mould?

Yep, looks like black mould from where it won't have got enough ventilation underneath. You can probably get it out with some work, but you'll just want to make sure it definitely hasn't got into the mattress itself.

Phew that is reassuring. Indeed it's just been slid in and out from under our own bed so the bottom wouldn't have had any ventilation. I'll try to get the marks out somehow.