Surely the problem is basically one of water.

All living things that live in water have immediate access to it. Those things that live on land have three options.

A. They move about until they can reach a source of water when they need it. Which is what most animals do.

B. They hijack the water they need from other living things that they take as food - again some animals do that also.

C. They transport water from the water table to the rest of their cells which is what trees and all other land plants do.

How can a tiny individual organism survive on land if it does not obtain the water it needs in one of those ways. If it is going to rely on A or B then it really needs to be mobile because trying to obtain the water it needs from chance encounters of passing organisms is a high risk strategy. A small organism could not hope to store rainwater for its needs either. It would need to be much larger in which case it would be plant (probably a cactus). If it is going to rely on C and draw water from the water table then it needs long transport structures in which case it is not going to be a colony of tiny individuals but a larger form of life like a plant. So I do not see how something like coral could grow on land without ceasing to be coral and instead becoming some form of plant (regardless of whether it caught and digested passing animals) Compared to seawater and pondwater the atmosphere is a pretty sterile medium for life.

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