X15 coolant leak (weird)

[LTWON]

I have a 2020 efficiency x15 that I rebuilt not to long ago, quick backstory on this engine is I bought it with a almost fresh Cummins overhaul but do to the techs incompetence that rebuilt it it ended up spinning a rod and needing a crank. This was in August of last year, I rebuilt it myself (cut counter bore at Cummins, stainless shims .0145 liner protrusion etc) I am leaking coolant at the back of the engine, now it’s not a lot but enough to annoy me. It’s on the driver side rear of the engine and it seems as it’s an external head gasket leak but it’s dry around the head and block mating surface. Has anybody ever had a freeze plug fail and cause a leak on the head. It’s not something obvious but definitely annoying. Also this just started happening, the engine has approx 15k since rebuild. The shiny residue under the head has been there since rebuilding it, so that’s not it.