Looks like cobweb mold. Probably not detrimental to your plants but may be a good idea to wipe down all your blumat lines with a 10% bleach, drop of dish soap, and water solution.
You obviously don't want this to spread to your buds. The soaker might be leaving it too wet for too long. I use drip style blumat carrots myself but never saw anything like this. Maybe change out that straw for something drier and better draining even though that is what I use as well. Change it out with some dry straw and/or try mulching with something inert like perlite or sand.