Is my blog controller too "Fat"?
The only thing I can see is moving the related/cleaned posts method into an instance method on Micropost. I'm also not sure what the implementation of Micropost.tagged_with is like, but I bet that could be moved into an instance method. Without that, I might suggest:

class Micropost &lt; ActiveRecord::Base
  def cleaned_related_posts(limit = -1)
    posts = Micropost.tagged_with(tag_list, :on =&gt; :tags, :any =&gt; true).reject { |p| p == self }
    posts = posts.take(limit) if limit &gt; 0
    posts
  end
end


Then your controller becomes:

def show
  @micropost = Micropost.find(params[:id])
  @tags = @micropost.tag_counts_on(:tags)
  @responses =  @micropost.responses
  @response = current_user.responses.build if member_signed_in?
  impressionist(@micropost)
  @cleaned_related_posts = @micropost.clean_related_posts 5
end


It's not really much of a savings, but it does at least centralize the logic for related posts. This controller isn't really fat to begin with.


  ... RubyMine keeps showing me a warning that the controller should call only one method.


That's just plain silly. IDE suggestions should be taken with a grain of salt (and a wedge of lime, and a shot of tequila).