Hopefully I can offer some help, however I must say I've not yet had to try this.
OK lime mortar is alkaline based, so by using a slight acid based liquid to soften it then it should wash off with just a light water stream. One of the safest and easiest acid liquids is pure orange, lemon juice or vinegar, wet the mortar and allow it to soak for a couple of mins then wash with a hose. You may need to do this a couple of times but it should work. Sorry I can't offer more exact help and as I said I've not had to do this so no guarentees.