Why is the sand in my paver joints green?

Before asking what the green is, look at where it sits: in the joint line, not on the paver face.

A joint is a low, sand-filled line between pavers, and water running across the surface collects there along with whatever it carries, such as leaf litter, seeds, soil and insect matter. That is why growth tends to show in the joint line before the paver face beside it. A slick, wet organic layer in a joint, the kind of thing the word biofilm describes, is a condition to notice, not a diagnosis of a specific organism.

Naples supplies the water. At Naples Municipal Airport, NOAA's 1991 to 2020 normals give 7.41 inches of rain in June, 7.77 in July, 8.89 in August and 8.60 in September, month after month of rain for a joint to hold.

Ask what the green actually is and the honest answer is that none of the sources cited here names it for a residential paver joint.

UF/IFAS Extension's publication on Nostoc in nurseries and greenhouses says Nostoc-like cyanobacteria “grow on the surface of soil, gravel, cement, and even plastic containers, producing macroscopic mats.” It says “these mats desiccate and become flaky during dry periods, but in the presence of water or any moisture, they swell to form thick, dark green, gelatinous masses,” and that the growth is “extremely slippery and wet.” The publication pictures colonies on sandy and limestone ground. That is an example of what a dark green, gelatinous growth can look like, not an identification of what is in any joint; identifying a growth is a question for UF/IFAS Extension, not for a cleaner.

Green on the flat face of a pool deck paver is a different question with its own page, answered on why pool deck pavers turn green. Getting an existing growth off the pavers, whatever it turns out to be, is covered on removing mold from pavers, and the chemistry for it belongs to the paver maker and the product label. Weeds rooted in the same joint are plants grown from seed, a separate mechanism answered on where weeds between pavers come from.

Two kinds of product are described as resisting what ends up in a joint, and neither one names a growth in it.

ICPI Tech Spec 5 counts the intrusion of organic matter, seeds and insects, with flowing water, wind and tire action, among the risks to joint sand that a stabilizer's primary function reduces. What joint stabilization covers is on its glossary page.

One maker goes further for its own product. SEK-Surebond formulates SB-1300 with an antifungal “designed to help inhibit the growth of mold, moss and algae on the sealer film,” and markets it for shaded areas and pool decks where mildew, fungus and algae are a concern. That is one maker's claim about its own film, not a statement about every sealer or about growth already in an unsealed joint. What the Naples wet season does to joint sand itself is covered under joint sand, weeds and rain on the paver cleaning page.