Hey,
You're looking at harmless salt buildup as the the water evaporates from the fabric.
No bid deal, rinse it away if you're concerned, but it shouldnt harm anything.
Cheers,
I believe this is just a build up of minerals from your nutrient solution and nothing to be too concerned with. Best to make sure your fabric pot is not sitting in any run off water for long periods of time. Best of luck! 😼