Should I fill out the store's incident report?

Reporting and completing their form are related but distinct.

Report the fall. Yes, always. That's what triggers preservation and documentation.

On the form itself, be factual and brief. Where it happened, when, what was on the floor, that you fell. Stick to what you observed.

What to avoid writing. Speculation about cause. Characterizations of your injuries before you've been examined — "I'm okay," "just shaken up," "my back is a little sore" all get used later. Guesses about how long the substance was there. Anything accepting blame, including reflexive apologies.

Ask for a copy. You may not get one, but ask, and note whether it was refused.

On recorded statements. Some businesses request one at the scene. You can decline, and generally should.

A practical point. Your description of what was on the floor may be the only contemporaneous account of the hazard. Be specific about the substance, its extent, and its appearance — those details support the duration argument later.