Dust accounts are accounts with very little funds, generally less than the amount needed to conduct a transaction. Too many dust accounts add unnecessary data to the blockchain, which would make it difficult for full nodes to sync with the network (since every full node has a complete copy of the blockchain).
On the Acala network, an address is only active when it holds a minimum amount (exact number TBD). This minimum amount is called “Existential Deposit” (ED), similar to ED on Polkadot. All native tokens e.g. DOT, ACA, aUSD, BTC etc. would have this feature, but it is not enforced upon ERC-20 tokens in the EVM.