Samsung T5 portable ssd and Mojave

Yes. Make sure the drive is formatted for Mac. Mac OS Extended (Journaled) if using Time Machine. You can format using Disk Utility. Note the warning: "Erasing a disk or volume permanently deletes all of its files. Before continuing, make sure that you have a backup of any files that you want to keep".

How to erase a disk for Mac

Types of disks you can use with Time Machine on Mac

Just a suggestion. Open Disk Utility. In the upper left hand corner click on "View". Then click on "Show All Devices". Select the Samsung T5, Choose Erase. Select "MacOS Extended (Journaled), with a GUID Partition Map. Click on Erase. This will reformat your T5.