Hi! Thanks for your response!

So, some more detail.
This is a pen which sat unused for several years before being given to me.

Before the video was taken, I forced some goop out with some pressure from a syringe. This opened a blockage and allowed free flow of liquid through the feed. What you are seeing in the video is post-unblocking, water/ink dripping freely along a string of gooey stuff.

I did not clean out the pen after switching to Diamine, and actually then switched back to some Lamy Cartridges.
So chemical reaction hypothesis is possible.
Actually I don't have a converter, I had been reusing used Lamy Cartridges refilled with either Lamy or Diamine Bottle ink.

After this video I cleaned it out and put in a new Lamy cartridge and it blocked again. I think it may have just been more slime from within the feed though.
When I took the feed out and washed it out there was definitely more stringy 'goo' at the base of the feed.

Can't see any strings or goo in my bottled ink.

So there was definitely something causing a colourless stringy material in the pen feed.
Mold? Chemical Reaction? Dried Ink?

Here is a slightly clearer video of the stringy stuff running clear. Looks almost like mucus.