They were probably tending the leafhopper also present in your photo. They protect the leafhoppers from predators and parasites, stroke them with their antennas and in return get to eat the sugary shit
ohhh. I thought the leaf hopper was doing a good job of hiding from the ants. I did see the passing ant stop above and look like it was feeling below, but then it quickly moved on. I didn't realise the leafhoppers were so small. No wonder I haven't seen one up till now!