It's not a deficiency, nor a sickness - it's a "sport". Just a branch of the plant, getting white. A lot of plants does this, but Pogostemon helferii does this quite often - we see them (too) often at Tropica, usually discarding them. You can't grow them on, since they allmost allways fall back to green in new growth very fast.
- so......if you don't like the white, pinch it off and the plant will allmost certainly grow new, green shoots!!
Personally I like it, both aestetically.....and as the "joke of nature", it actually is.