Pretty terrarium on the shelf :) but asparagus fern’s tips are turning white?

Hi all! I made this big pretty boy a couple of months ago and it lives in my shelf with a grow light lid. I’m pretty happy about it 🙂 But my asparagus fern’s tips are slowly becoming white.

Any ideas on why that could be?

Moss is growing well, fitonias too, so I’m guessing humidity is right. Light is on for 12hs/day and the lid doesn’t close completely, but almost. I’m in London and it has been pretty hot these days. Although the terrarium doesn’t get any natural light, I’m thinking it could be the heat?

Ps: I know the visibility of the container is not too clear, so the pictures aren’t the best, but that’s because I made this out of a whey protein big tub! Still need to remove the label glue a bit better.