To stop losing coolant, you need to figure out where it’s going.
I am sure the reservoir can be refilled by the end user. But given there’s a mystery as to why you need more coolant to begin with, I’d wait for GF support to respond with a recommendation.
That being said, the bubble doesn’t look that bad to me. If the level of the coolant was low enough that the inner laser tube was at risk of being exposed to air I’d be worried. But you’d need to lose half of what’s in the system to get that low… you would definitely notice it leaking out long before then.
I wouldn’t stop using the machine based on that image. I suspect it’s just the original fill was a little low.