It this a stromatolite fossil? If it is, it’s probabaly at least 1 billion years old
No, it is not a fossil.
This is a sedimentary rock with veins of a stronger material in it.
For example, it could be limestone with veins of quartz, or mudstone with veins of calcite.
Doesn't look like a stromatolite... The layering would be more intense, more 'crumbled'... Stromatolites form by algae mats and sand layers 'catched' by the algae. After a sand layer, a new layer of algae forms on top of it. Keeping this in mind, your piece looks quite different. I saw dozens of fossil stromatolites and I never encountered one looking like your piece.
Like Gimelist stated, the veins of harder material shouldn't be there, too