Hello, I am a solicitor with 20 years experience. I will try to answer this for you.
Your landlord has no right to do this. You should tell him to desist. As long as your washing habits are not damaging the property or causing nuisance to the other tenants it is none of his business.
It is probably too early to complain about harassment and there is not really any effective legal remedy for this sort of relatively low level harassment.