This usually happens on Binance and is caused by the fact that Binance deducts the trading fee from the amount of your open position. That's why there will always be some funds left (called dust) when selling a position.
Solutions
When on Binance, you can configure to use BNB for trading fees. This could solve the issue. A requirement is that you own BNB on your exchange account. An extra benefit is that it will save you 25% on trading fees.
Another solution is to use the "Convert dust to BNB" feature that can be found on your Cryptohopper Dashboard under: Current assets > Wallet scrubber > Convert dust to BNB
