Stash Invest accounts are taxable brokerage accounts and you are required by the IRS to report income earned from capital gains and other applicable distributions. Each year, Stash will send you a tax statement so that you can file your taxes appropriately.
All income is required to be reported to the IRS even if a statement is not received. You most likely should have received a 1099, it may possibly be online on your account if you sign in.