Matrix Field Slug Output I am not sure why, but I cannot seem to get the short name/handle/slug of a matrix field that is a category field.  I need to get it's slug.

{% for row in entry.matrixField %}

 {{ row.matrixChild.slug }}

{% endfor %}
That is returning an ElementQuery. I'm assuming you only need to get the first category. You need to use .one to get the category element.

{% for row in entry.matrixField.all() %}
    {{ row.matrixChild.one.slug }}
{% endfor %}


If you wish to display more than one category you can use a {% nav %} or {% for %} to loop through them. See the category docs.