CW, you did the right thing reporting that find. Given your description, the find’s proximity to the old Fort Pierce Naval Amphibious Training Base and the use of WP in WWII...better safe than sorry.
See 2017 article below describing a similar find on a beach in Germany.
German woman picks up WW2 white phosphorus bomb after mistaking it for amber
Walker on banks of River Elbe near Hamburg put the object in the pocket of her jacket which later burst into flames
Jeff Farrell Sunday 6 August 2017 09:38 BST
UK Independent
A woman in Germany narrowly avoided injury after she picked up a shiny object on a beach she believed to be amber, but which turned out to be part of a Second World War bomb that later burst into flames.
The 41-year-old was out walking on the banks of the Elbe river near Hamburg when she plucked the small stone-like item from wet sand and put it in the pocket of her jacket.
But the souvenir turned out to be a centimetre-long chunk of white phosphorus – which ignited just moments later in the incident near the town of Wedel.
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https://www.independent.co.uk/news/w...-a7879121.html