I did some further online research. That’s what I found:
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Mold on cards usually doesn’t show up as clean geometric shapes. It’s more like irregular speckling/spotting, sometimes powdery/fuzzy “colonies”, and can be white/gray/green/black, often clustered and uneven.
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These tiny square/rectangular “artifacts” in PSA slabs are observed by many collectors:
Eliteforum: Link
Collectors Forum: Link -
What you see are most likely holder imperfections, not the card. Another E4 member mentions that these are most likely plastic injection-molding / encapsulation artifacts, e.g., a small inclusion where a pellet/fragment didn’t fully melt or a minor flow/pressure defect left a square-ish mark in the plastic.
Later tonight, I’ll take a few comparison photos of my own slabs that show the same issue and share them here.