To ensure the rolling ball algorithm in ImageJ is working, first make sure your image is properly converted to 8-bit and that the rolling ball radius is set appropriately
https://8ballpoolapkmodx.com/8-ball-pool-mod-apk-for-ios/ . For cells of varying sizes, the radius should be at least as large as the largest object you want treated as background. For example, if your biggest cell is about 110×130 pixels, try setting the radius slightly larger, like 130–140 pixels. Choosing the white background option inverts the process, which can make the image appear very different.