ah yikes, yea looks similar to what took over and killed my last grow. I was using just hydroguard at that time too, so decided to go the tea route to get the maximum strength and variety of the bennies.
Basically used the tea here:
https://www.rollitup.org/t/dwc-root-slime-cure-aka-how-to-breed-beneficial-microbes.361430/
I bubbled it for 48hrs total, but I did take some tea at 24hrs, used it directly, and mixed some back in to the 48hr tea (keeps mycos alive). I'm also using the mycos directly on the plants since germination because it takes a while and a lot of effort for them to get established in DWC plants.
This is what I used for mine (there are plenty of alterations/modifications possible):
Tea:
* Floralicious plus
* Earth Juice - Rooter's Mycorrhizae
* HydroGuard
* Age Old EWC
* Earth Juice - Hi-Brix molasses
One of the biggest rot contributors seems to be adding anything organic to the rez directly. I think the floralicious plus is what screwed things for me last time. Even the small amount of dead root matter is usually what gives root rot a head start (plants might kill off roots under stress), then it can start actively destroying roots for new food. So with the tea, if you want to use any organic additives for the plants (in my case, humics and kelp via the flora+), it seems best to mix it in as part of the tea making process. That way if its going to benefit any microbes, it'll be the ones we selected intentionally, and they can get a head start breaking down the organics for use by your plants. Then when you add it to the rez and things start to 'liven up' a bit, any bad micros will be overrun and starved out by the billions of bennies. The good bennies will 'clean' the dead matter off of your roots, and basically destroy any food source that would have given a 'leg up' to the bad microbes.
The Mycorrhizae definitely seems to be the coolest. I read online that sometimes the fungus will 'kill' a seed before its germinated, saw that happen to 2 (the seeds turned into a white fungus), but the remaining ones will basically have this fungus living inside and outside of their root system. The tea itself isnt great at sustaining Mycorrhizae life, so inoculate the seedlings separately and rather often. I just mix some with some distilled water + a bit of tea, fridge overnight, and apply it directly to the top of the rockwool every so often. Mycorrhizae seems kinda easy to kill tho, so I only make one batch of the inoculant at a time.