content_tag populate drop-down with unknow "black squre" in front

I used the following code (in .slim file) to generate a dropdown. Each item in the dropdown is a hyperlink to other website. But I could not find out where does the black square ahead of each item is coming from. How to remove it?

li.dropdown
     a.dropdown-toggle data-toggle="dropdown" href="#"
         | Other 
         span.caret
     ul.dropdown-menu role="menu" 
        == content_tag :ul do items.map do |item| concat(content_tag(:li) do link_to(item, "#") end) end end