For Removing decals: The eraser functionality can be used to remove decals, however do note that this is not a logical “Remove” operation; Erasing simply scrubs the pixels painted around a given location. If you want to easily remove an entire decal via logical mapping (i.e. “please remove the entire decal I painted approximately around this location”) then you’ll need to use your own logic to support that. A simple technique to achieve this would be to maintain a custom list of all painted decal locations (and their brush size) and erase all decals that are sufficiently close to the brush location from which you desire to remove decals.
For Moving decals: This could work by erasing the previous decal and placing a new one each time the cursor position moves sufficiently. As you’re only moving one decal at a time, you just need to memorize the last painted decal location (and its brush size) and erase from there the next time. The performance cost of this will vary depending on your target mesh type and UV method (a.k.a painting workflow; see this kb article for more).