What's this and how to clean it?

[Cantdeal5]

Stripped my DS' bed and thought I'd wash the mattress cover as it's a nice day. This was on the mattress underneath. Is it mould?

I'm so confused as our house isn't mouldy at all. I feel like a terrible parent for letting him sleep on that.

Its a fairly new mattress so the only thing I can think of is that last time I washed the cover I put it back on before it was fully dry.

How shall I clean it without making the mattress even more wet?

Is it possible the child spilled something on there and it went mouldy?
Do you have a steam cleaner? That would kill the mould without making it too wet, won't get the stain off though. You'd probably need bleach for that, which would kill the mould anyway.

Dr Beckmanns carpet shampoo is fab for mattresses. It has a brush on the end and you just scrub it in.

www.dr-beckmann.co.uk/products/carpet-stain-remover/