tinymce-docs-4x/_includes/configuration/custom-elements.md

custom_elements

This option enables you to specify non-HTML elements for the editor.

This way you can handle non-HTML elements inside an HTML editor. You can prefix the element names with a ~ if you want the element to behave as a span element and not a div element.

Type: String

Default Value: div elements

tinyMCE.init({
  selector: "textarea",  // change this value according to your html
  extended_valid_elements : 'mycustomblock[id],mycustominline',
  custom_elements : 'mycustomblock,~mycustominline' // Notice the ~ prefix to force a span element for the element
});