Burst pipe caused mold and insurer is trying to split the claim to deny the mold part

We had a pipe burst in our basement wall in January. By the time we found it - maybe 36 hours after it happened - there was significant water damage and what turned out to be pre-existing mold behind the drywall that the water apparently accelerated. The remediation company found about 40 square feet of mold, all behind the wall, not visible from the surface.

My insurer initially sent an adjuster who wrote up a combined estimate of about $28,000 for water damage and mold remediation together. Now, three weeks later, they came back and said they're splitting the claim into two separate claims - one for water damage (approved) and one for mold (denied). Their position is that the mold was "pre-existing" and not caused by the burst pipe, even though the remediation company's report says the moisture levels in the affected area are consistent with the recent water event.

My policy does cover sudden and accidental water damage and has a mold endorsement for mold resulting from covered water damage. I've read that section four times and I genuinely believe my claim is covered. But I have no idea how to fight a claim that gets split like this. Do I hire a public adjuster? An attorney? File a complaint somewhere?