Hi mate, Looks like calcium deficiency due to excessive nitrogen. Exactly what's going on with my plants. I've been trying to correct my nitrogen toxicity for a few feeds with little success. This has started with my plants today, a little less obvious than yours though.
You need to cut the N down a little and feed some cal mag. Careful with the calmag as most seem to contain nitrogen. I'm going to be cutting all sources of N on my plants until they start to turn a little lighter in colour. (Not cutting the calmag obviously)
Hope this helps 🙂