This study showed the eliciting effect of aqueous extracts from Ascophyllum nodosum on the development of Eruca sativa microgreens in a quartz-based substrate. There is no information in the literature on the impact of its use on the quality, bioactive compound content, and nutritional value of arugula microgreens. Assays that have been made include the following: total phenolic content (TPC), total flavonoid content (TFV), enzymes such as phenylalanine ammonia (PAL) and polyphenol oxidase (PPO), and enzymes involved in the scavenging of reactive oxygen species such as catalase (CAT) and superoxide dismutase (SOD). The antioxidant activity against DPPH (2,2-diphenyl-1-picrylhydrazyl) was also evaluated. The total phenolic content of arugula microgreens increased significantly after an application of aqueous extracts of A. nodosum to the substrate. The greatest increase in flavonoid content (89%) and antioxidant activity against DPPH (82%) was observed at a concentration of 2.5%. The highest increase in activity of CAT (68.2%), SOD (25%), PAL (13-fold), and PPO (84.2%) was observed with the application of 5% extract. In conclusion, the use of Ascophyllum nodosum algae affects TPC and TFV, antioxidant activity, PAL, PPO, SOD, and CAT in the microgreens studied. By prioritising organic and environmentally friendly extracts, growers can contribute to a more sustainable and healthier food system, especially in the case of the production of Eruca sativa microgreens.