How do you seal travertine floors?
Travertine can have many impurities, and being a sedimentary rock, could have a nugget of softer material.

In any case, use a stiff nylon brush and remove the chalky area.

Rinse the area and note if water absorbs in the nugget area..
If so, good; grout and/or sealer will attach securely.

I would recommend a bit of matching grout in the divot, followed by a penetrating stone sealer.(I prefer Miracle products, but others are fine).