The SEG is a metered export payment that large energy suppliers (with over 150,000 customers) are obliged to pay to their customers.
The obligation came into force on 1 January 2020 and means that suppliers must pay customers a pence per kilowatt-hour value for exported energy, that is above 0p/kWh.
Levelise is not a SEG provider but you can find a list of suppliers here.
If you haven't already set up a SEG tariff, you will need to do so in order to get paid for your exported energy. This does not impact and wont impact the payments you get from us for Grid Services.