Is this nitrogen toxicity?

I'd give it a second.. that's extremely minor and could be environmental or other causes not related to nutrients. If it were a tox of N, it'd also start getting darker below and rise up the plant over time. you can always put an extra water-only irrigation between your application of seaweed. if it continues to progress, this is probably best -- or reduce what you give in any one sitting and retain frequency... reduce it one way or another if it continues, but i'd wait and see at this point. based on node spacing, i'd say reduce feed a bit, or increase light intensity. if you increase intensity, it'll eat more (assumes heat issues are not caused etc.. could be a bad idea too in some contexts). your node spacing looks like it can use a bit more. try to find another 30-60mins of intense sunlight for it? incrementally increase it and see if it is a positive impact, if possible.