Windows, sill going mouldy

I have a fairly settled HMO with 5 bedroom. It is the usual attention intensive, repairs and occasional bad tenant slipping through the net, etc. But otherwise good.

Since lockdown, I have noticed almost all the bedrooms windows & sills are showing increased signs of mould especially in the bottom half.

Has anyone.had this problem and does anyone have a solution.
I want to nip this in the bud please.

You have to remind tenants that ventilate home and not to keep the windows and windows sill clean and dry.
As far as I am aware tenants should take care.

You gave the property in good condition and maintaining is their responsibility.

Mould will develop if there is condensation because of drying clothes indoor, etc

Building up mould in shower and bathroom is another problem when tenants do not keep drying after showers and bath.

I meant to say please to NOTE to keep windows and windows sill dry and clean and ventilate home.

Thank you Prafula.
That was pretty comprehensive advice

Just learnt. Each tenant has bought a kettle and taken to making their own tea/coffee in their rooms.
Should I stop this practice? If yes, how do I enforce this. I do not live in that house.

Absolutely stop the practice. Where does that water go? It stays in the room. Buy a humidity meter £5 and tell them it needs to be kept in check; and they will be liable for any repairs due to their habits