Call Indiana's statewide Adult Protective Services line at 1-800-992-6978, or file online. Indiana APS is state-run through five regional field teams under FSSA, not county-run, so there is no separate Marion County number to look up.
What to do next
Because APS is organized as five statewide regional teams rather than by county, families searching for a 'Marion County APS number' or a 'Hamilton County APS number' will not find one -- and that is expected, not a gap in this site's research. The same statewide line and online form route every report, regardless of which of the six counties this site covers the situation occurred in. Reports can be made anonymously, and Indiana law does not require you to have proof, only a good-faith suspicion, before reporting.