yes it looks to be the bacterial prevention icon.
Most hot water systems are compatible with the Nest thermostat’s Bacteria Prevention mode, so it’s recommended that you keep it on. But you can turn it off if you’ll be away for an extended time, such as if you have a holiday home you only use during the summer.
1.Press your thermostat ring to bring up the Quick
I have two new Generation 3 thermostats I have just installed They have both been set up identically with Combi Boiler heat with on/off. There is no hot water service connected. One display shows the green Eco leaf symbol and the other the yellow water drop Bacteria prevention symbol. I can't turn of the yellow Bacteria Prevention symbol as there is no option under the equipment setting to do so. How do I remove this symbol?