That news article is a bit light on details, so here's a link to a PDF of the paper itself: https://ntrs.nasa.gov/api/citations/20250001217/downloads/Hurowitz et al LPSC.pdf

Poppy seeds and leopard spots co-occur with organic carbon in ferric-iron poor mudstones but are absent in ferric iron-rich mudstones and conglomerates that lack organic matter (Fig. 1C). These relationships indicate the vivianite and greigite in these features formed by reaction between organic matter, oxidized iron, and sulfate; geological context and petrography indicate these reactions occurred at low temperatures. On Earth, such organic matter mediated mineral forming reactions are often driven by, or are closely associated with, microbial respiration of organic matter [e.g., 5], and models of the reaction chemistry that formed these features are consistent with microbially mediated process [6]. These properties mark poppy seeds and leopard spots as potential biosignatures. Analysis of the core sample collected from this unit using high-sensitivity instrumentation on Earth will enable tests to assess the biogenicity of the minerals, organics, and textures it contains.