This is a replacement plastic 'nut' that you use to lock the replaceable sponge heads to the mop.  There are four nuts, one in each corner, and they can break easily.

Dimensions:

• Hexagonal total diameter: 32mm
• Hexagonal thickness: 5mm
• Nut Tip (the bit that locks inside the sponge head): 12mm x 16mm
• Nut tip depth: 7mm

Printer Settings:

• PLA (other filaments should be OK)
• Standard Quality (0.2mm)
• 40% infill
• Supports enabled