that would be a pretty complicated collision mask... but it should work.
an alternative would be to make simpler cloud shapes, and then combine several to make up a cloud that looks like that - an if you randomise it, you could get different clouds each time.
I found the problem, somehow the collision mask are not correctly set. So the clouds are getting a full rectangular collision box...
Combining with your solution it works perfectly now.
Thanks again for everyones help.