It doesn’t mean your house is being treated as a marital asset, so try not to panic.
In financial remedy cases they look at the overall financial position of each person, including their living situation. Because your partner lives with you, the mediator is likely just trying to understand his housing circumstances rather than suggesting your property is up for division.
From what you’ve said, the house is in your sole name, you bought it before the relationship, and he hasn’t contributed financially. That would usually make it non-matrimonial, so it wouldn’t be something his ex could claim a share of.
However, it can still be taken into account in a different way. For example, his ex might argue that his housing needs are already being met because he’s living with a partner who owns a property. That doesn’t give her any rights over your home, but it can sometimes affect what he might receive in a settlement.
The fact they’ve only asked for a value (and not equity) suggests it’s more about understanding the situation than trying to include it as an asset.
He just needs to be clear that the property is entirely yours, he has no legal or beneficial interest in it, and there’s little or no equity anyway, especially with the loan attached.
You’re not a party to the case, and the court can’t make orders against your property. This is more about context than entitlement.