any clue?

All my Pogostemon helferi tops melted there is a hard stem left. I do have new growth branching out at least 2-3 per plant. One of my Lugwigia glandulosa is doing the same thing, but I have at least 12 plants branching off of this one stem.
The melt off was even in a slime form, that I removed from tank with a 50 percent water change.
Any clue what would cause this or what I could test for?
I can't post pics Ive been slowed down for a week. But I can answer any questions needed. Thanks.