What is growing on my plants and how do I treat it? See details on this photo:
That's honeydew. It's a sugary thing and it is excreted by scale insects and aphids. Wipe it off and spray the plant with insecticidal soap.
Well it looks rather like a woolly aphid infestation to me - it would have helped to know what plant it's present on though, to help with diagnosis. Depending on how bad and widespread it is, you can spot treat with neem oil or insecticidal soap, or find an insecticidal spray to treat with - but if its fairly minor, you often find other insects coming into eat the aphids anyway. If this is an edible plant, be careful if you choose to use insecticide, it should be one that's suitable for use on edible plants. If it's widespread, and whatever plant it is doesn't mind being pruned, you can cut out the worst affected branches. Info here https://www.gardeningknowhow.com/plant-problems/pests/insects/woolly-aphid-control.htm. Note the insecticide recommended may not be available in your area.