Getting free bitcoin with moon bitcoin cash is legit. The Moon group of websites are crypto faucets, which means they will give you free tokens just for showing up and clicking. While the tokens are free, you will only be given a very small amount, and you will also have to sign up for a CoinPot microwallet.
Once you have registered for a CoinPot microwallet, and also created an account on whatever Moon faucet you want to use, you can start clicking and verifying that you are human to receive free tokens.
There are Moon faucets for Bitcoin, Bitcoin Cash, Dogecoin, Litecoin and Dash. You will have to register on each site separately, but the process is simple and only requires an email address.
While the tokens you will receive on the Moon faucets are 100% free, you won't be getting a large amount each time you click. The Bitcoin that is given away is measured in Satoshis, and one Satoshi is worth 1/100,000,000 of a Bitcoin.
If you are using the Moon Bitcoin faucet you will probably get between 5 and 20 Satoshis every time you are given the opportunity to gain free tokens, and the longer you wait to use the faucet, the more you are likely to receive.