An Echo knight's echo is an object, which makes it hard to turn invisible, as invisibility and greater invisibility only affect creatures. What options, from any non-homebrew source, allow you to turn an object (thats not being worn or carried by an invisible creature) invisible in 5e?
Dust of Disappearance will work.
As an added bonus, it both specifically says that it applies to creatures and objects, but the invisibility is only broken prematurely if a creature affected by it makes an attack. So your Echo will stay invisible for the entire duration.
A second option is the Sequester spell. It's pretty high level, but makes objects invisible. As with the Dust, it only puts creatures in statis, so your Echo will be invisible until it's dispelled or it takes damage.