How do I Add HTML to the_excerpt() & the_content() Output?

get_the_excerpt() returns the excerpt, but only after applying get_the_excerpt filters to it. This causes infinite recursion, as your filter handler will be called to an infinite depth.

Of course, the best solution is the css one (see @spacegrrl’s answer), but if you have your reasons for keeping that <span>, please note that the excerpt can be passed as a parameter to your handler. Just add an $the_excerpt parameter and use that, instead of calling get_the_excerpt().