Market cap = price x money supply
You must have misunderstood the trader.
I guess your formula is partially correct but it has to be
Market cap = price x
Number of Coins in circulationThis is in case of crypto currencies. Moreover, even if you have to calculate it for a company value then it has to be prices x shares of company in the market.
What do you mean by money supply exactly ?
@OP so it is anyway clear that market cap is actually driven by the change in prices or circulating supply of the respective coins.