Your money at Merrill Lynch is generally quite safe from a structural standpoint. Merrill is part of Bank of America and a member of SIPC, which protects up to $500,000 per customer (including $250,000 in cash) if the firm itself fails, though SIPC does not cover investment losses from market declines.
Merrill also carries additional supplemental insurance through its clearing broker for balances above SIPC limits. Cash held in brokerage sweep accounts is often moved into FDIC insured bank deposit programs for extra protection, up to standard FDIC limits.
The real risks to your money are market risk (investments can lose value), advisor-related risk (fees, unsuitable recommendations), and your own asset allocation choices, not custodial safety. If you want specifics, tell me what accounts or holdings you have there.