Methinks there is no fossil here, though there is a hint of echonoderm ambulacral plates.
The 'cells' are the rock, possibly an iron-rich siltstone, and the white parts are veins, possibly of calcite (if they were quartz, they would probably stand out from the siltstone more prominently).
In some places the siltstone has fallen out, leaving a socket rimmed by the vein mineral.
This sort of structure arises either by:
- the rock fracturing, leaving irregular voids, which later become filled with vein minerals
- in-situ chemical processes happening as the sediment turns to rock and subsequently, resulting in nodules or concretions.
Some of the latter can have a rather regularly fractured structure, and if there is prominent colour contrast between rock and vein (eg.iron-rich siltstone with white veining), such specimens can be quite eye-catching. The regularity often leads folks to think they are fossils. I remember seeing one in a local museum in the west of Ireland where such a 'septarian nodule' was labelled as a fossil turtle.