Market cap = Current price x supply... it is a common misconception that the market cap is representative of dollars invested.. its highly under this amount.. for example Bitfinex has been dumping 2-3k bitcoins causing these bart candles patterns we've been seeing over the last couple of months... 2000 BTC is only curently worth 14 million, but if you sell these coins at market (along with arbitrage bots picking up on this data and correcting on other exchanges.. this one 14 million dollar sell can wipe 2-5 billion dollars off of the market cap. A lot of people don't understand this correlation.
Yes marketcap is like you said Current price x supply only. Like on stocks. But there is something that is not like on stock and im talking about that. Im talking about first factor of this mathematical formula.
Current price is most often made by other cryptocurrencies (mostly bitcoin) instead of $. Made from the begging. From ICO (which suports btc eth) till now (most of volume made by btc-eth/alts).
You have bitcoin pumped by $ to 110 bil$ (not pumped by 110bil$ but pumped to 110bil$ with unknown amount of $) and other currencies pumped mostly by bitcoin to another 190bil. instead of 300bil $ market pumped by dolars. That gives us circle of pumping. I mean that if only hypothetical you will close every possible way to invest $ into market and pull $ from market marketcap will continue to grow becouse new coins will be bought from old coins. And its price will be calculated through old coins. If you will do that on stock that not a single price would change and marketap will stop too.
Im sorry if i wasnt clear enought in first post.
Thats why total marketcpa is decrising that fast. Its not only caused by speculation but also because every altcoins price is based on bitcoin from begining till now