FDIC and SIPC are different types of insurance that cover different things.
FDIC insures cash in a bank account. With an FDIC-insured bank account, if you put $1000 into your account, you are guaranteed to get $1000 out (not including bank fees).
The “cash” you have in a brokerage account such as Robinhood, however, is not in a bank account. Instead, it is invested in a money market fund. A money market fund is a mutual fund that invests in high-quality short-term debt. It is considered very safe and very liquid, and many people consider it as safe as a bank account. However, it is possible for a money market fund to lose value. This happened to some money market funds in 2008.
SIPC does not cover investment losses, so if the money market fund that your brokerage uses loses value, it will not be guaranteed by the government. What SIPC does cover, however, is bankruptcy or misdeeds done by your brokerage.
To answer your question, the “cash” you have in your Robinhood account is insured against Robinhood doing something wrong, but it is not insured against an investment loss of the money market fund. It is possible (though not likely) to lose money in your account.