You can convert the entire document to grayscale when you export to PDF, but you don't have much control. Best method is to edit the origianl art, save a grayscale version, and relink.
If it is NOT for print, but online only you can fill the frame with black and set the image to "Luminosity" in the Effects Panel.
Rorohiko makes a great (and cheap) plug-in that allows you to do this and it's been updated for the latest version of InDesign. You can download a copy here https://www.rorohiko.com/wordpress/indesign-downloads/color2gray/.