Have you tried an analyze particles step?
Also are you sure you don’t mean inside of the selection, clearing the whitespace? Or do you really want to get rid of the tissue?
Either way, to generate an approximate ROI using your modified image, I set it to 8bit, Smoothed it twice, and then used Analyze Particles with the settings shown.
Other posts should be able to help you transfer the annotation back into the original image if you need help with that step. Here I used Add[t] in the ROI Manager to manually add the automatically generated ROI.
Alternatively, if you want some manual control, you can right click on the Wand Tool and use that with a particular Tolerance. Here I chose 22.
And here recoloring the wand tool selection as Cyan.