Yeah they are legit, I found this on brokers check at FINRA https://brokercheck.finra.org/firm/summary/288202
Registered securities broker
Alpaca Securities LLC is a member of Financial Industry Regulatory Authority, Inc.
You can check the background of Alpaca Securities on [FINRA’s BrokerCheck]
Alpaca Securities is also a member of SIPC - securities in your account are protected up to $500,000.