In PowerPoint, to make a Slide Master element (i.e., background element) appear on top of an element on the slide AND remove the placeholder icon, you will need to create a placeholder, then follow this work-around:
To have a master placeholder with no visible icon: Go to View>Slide Master>Master Layout>Insert Placeholder>Smart Art. Draw the desired shape and fill with solid color, pattern,or some other identifying trait (this will ultimately not be visible on the slide.) Close Master View. Then on slide, click the Smart Art icon, select any Smart Art, and “Cut” all elements from the Smart Art. Select the placeholder, right-click and choose "Format Object". Fill the shape with final color, image, pattern, etc.
Be sure the placeholder is stacked properly on the Master Slide so that it appears on the slide as expected. To change the order, go to View>Slide Master>Home>Arrange>Order Objects. Exit Slide View. The newly created placeholder will appear above anything added in Normal View, but in the correct sequence of all master elements.
To help future users of the template, on the Slide Master, add the following text to the newly created placeholder: "Do not change this element. If it gets changed on the slide, simply reapply the master." It may be helpful to also add the above instructions, in case the placeholder needs to be recreated.