I want to stop this specific strategy and keep the trades open
Open the IG Platform, make a change to a limit or a stop value shown in the respective Open Position. Then as soon as you click on a blank space on the page (thereby accepting / confirming your change) the System / Algo will stop but the trade will remain open as a manual trade.
If this is critical, think about what you are doing re changing the value of a Stop or a Limit Price.
To be safe … try above procedure on Demo Account first.