Is this just wear and tear?

Tenancy Deposits

Hi. My tenant has left today and the hemnes day bed mattresses are very soiled on the zip up covers and there was a bit of what looked like mould on one. It was very stuck to the mattress when I undid it, making me think it had got very wet maybe with a sugary drink. Do you think I should deduct the cost of new mattresses from her deposit, or would you say it’s just wear and tear? Personally, I feel I should charge her. There was no need for them to be as dirty as they are. Also, with Covid, it would be unhygienic to sleep on them even after thorough washing, I feel.