How to make a Moving Projectile?

Probably not bodymovers; something more like bodyforces or something that applies actual thrust to the grenade would suit you better and be more accurate. I don't know what you mean by 'prefab', but this is probably the way to go.

For example; if you wanted to throw the grenade where your mouse is pointed, your code would probably be something like this:

tool.Activated:Connect(function()
    local grenadeClone = fakeGrenade:Clone() -- fake grenade is probably the actual body of the grenade somewhere else like in replicatedstorage maybe (you could probably copy the handle of the tool)
    local bodyForce = Instance.new("BodyForce")
    grenadeClone:SetPrimaryPartCFrame(tool.Handle.CFrame)
    tool:Destroy()
    grenadeClone.Parent = workspace
    bodyForce.Force = playerHumanoid.TargetPoint.Unit * 100
    bodyForce.Parent = grenadeClone
end)

Remember: this is a rough draft; and probably won't work. Experiment.