What it means that a stablecoin is Algorithmic?
They are non collateral stablecoins, they are not backed by anything, an example is Ampleforth (AMPL). Such stables coins only depend on circulatory algorithm in which more of the coins are issued when price increase, and if the price of the coin is decreasing, some are taken off the market in a way their prices are somewhat stable at a particular price.
Is it possible to have decentralized algorithmic stablecoins?
Yes, like the Ampleforth.
Can we say CBDC's Central Bank Digital Currencies is similar to Algorithmic stablecoins
No, CBDC are collateralized with fiat, they are fiat backed stable coin while algorithmic stable coins are not backed by anything.