They're trying to host the mushrooms, because they do not have an anemone to host. Clowns will typically mouth their hosts to allow their bodies to get used to the anemone.
Personally I would cover up the mushrooms, with some, like half a strawberry container or something. Anything that will allow light/flow. This should deter the Clowns from the 'Shrooms.
What is your lighting like? Anemones require a decent amount of light.