In recent years, the incidence of breast cancer has increased significantly; therefore, much attention is being paid to research on natural plant-based raw materials in the treatment and prevention of cancer as well as in the treatment of antibiotic-resistant infections. Therefore, Chlorella vulgaris algae extract and indole-3-acetic acid (IAA)—a plant hormone with potential anticancer and antimicrobial properties—were selected for the study. The main objective was to evaluate the effect of algae extract and IAA on the proliferation of cells from three different breast cancer lines: MCF-7, ZR-75-1, and MDA-MB-231. In addition, an analysis of apoptosis and oxidative stress parameters in cancer cells was performed, as well as an assessment of IAA toxicity towards E. coli, S. aureus, and C. albicans. The results obtained allow us to conclude that the extract is effective against estrogen-dependent cells, while the effect of IAA alone varies depending on the microorganism studied, the cell line analyzed, and the concentration used. The extract in selected concentrations induces apoptosis and activates oxidative stress mechanisms, while IAA exhibits cytotoxicity at higher concentrations and stimulates proliferation at lower concentrations. This indicates the need to investigate the mechanisms of action of both Chlorella vulgaris algae extract and IAA in cancer and bacterial cells.