what is this?

Noticed this on a couple plantsView attachment 2403381 recently.

Not exactly sure, looks like either nutrient "burn" or nutrient deficiency...what are you giving it? Is it a hydroponic grow or soiless mix? I have a check list of things to look for, like checking ph level and such. The first thing that stand out to me is that It may need a good flush and then start gradually re-introducing of trace nutrients...hope this helps.

Is this the topmost part of the plant?

Id assume its a soil grow from the pic tell me if wrong. Looks like you may have a ph issue. What is the ph of the water with nutes going in and what is the ph of the run off water coming out after?

I'm using soiless potting mix. We have really hard water here, so untill I get a filtration system I'm buying gallons from the local grocery store. My ph stays right around 6. I'm using Aqua Vega mix (0-3-4) and (5-0-3) , Sweet and Liquid Karma, Rhizotonic, and B-29. All are used as per direction on bottle. The burn or whatever is at various growth stages on different plants... That picture show is about mid plant . I'm wondering if I'm doing too much in the way of nutes. Plants are at 6th node and about 18"

have u tried opening the tops of the containers to release the chlorine? GL

Check the water bottles.. a lot of your bottled water is purified with RO.. If it is, then it's completely stripped of any trace elements.. Using RO'd water requires you to use a lot af diff. additives.

I leave the tops off the water for at least 24hrs, and it is straight spring water. You don't think its too much nutes?

I don't think it's too much, how about giving more pics, of different parts, and farther away from her.
It does look like a deficiency, not a burn, but strange.
If you're soil-less, 6.0 is OK, a touch high, but still ok, but that is some weird looking stuff.
You said how tall they are, but didn't mention age, or if you're flowering or what.
More info = better answers!
View attachment 2403844View attachment 2403845