These photos were taken on 30th and 31st March at HortPark's Native Garden.1st photo: scales
2nd - 3rd & 5th photos: mealy bugs
4th photo: sooty mould
Simply put, the pests are all sucking insects and they are the primary infestation. They would suck the plant sap or juice and secrete some 'honeydew' like substance that would attract ants. Hence, they have this mutual beneficial relationship and the ants are usually sucking pests farmers.
Associated with the primary pests is usually the secondary infestation of fungal disease, like sooty mould. So in order to get rid of the sooty mould, one needs to remove the causal agent or the source of the problem. Otherwise, the effort would likely be futile.
Some links on the pests and the natural enemy of sucking pests, the ladybird, are shown below:
