Problem with first ferment - top layer discolouring

Hi folks,

I have experimented with a ferment at home. It is 3 liter glass kilner jar, so I can see the colour and activity.

It is jerusalem artichoke, some turmuric and other spices, and salt (amount as per a recipe I found).

The batch bubbled nicely, tastes fantastic and has a lovely vibrant yellow colour from the turmuric except for the very top layer, which is discoloured to a grey/green. I believe that is has oxidised, and looks unappetising.  I removed the top chunks and closed it up again. By morning the fresh top layer had discoloured again.

1) I the discoloured layer safe to eat?
2) What should I do differently to ensure future batches don't discolour?

I love ferments generally, and want to keep going, but need a little reassurance.