Alkalinity is the buffering capacity of the water, its ability to maintain a stable pH in the presence of acid (in the case of an aquarium, this means acids generated metabolically). It's a simple and cheap test, and gives you a good idea of the general condition of the water.
How often are you doing partial water changes? As far as I can tell, your system has no nitrate export, like live rock or macroalgae, so the only way for nitrate to leave the system is via water changes. I'd be willing to bet you have significant levels of nitrate and phosphate, and the slime algae are just loving it.
The low Ca is also probably a sign that the system needs more frequent water changes.
You might consider some kind of phosphate absorber, like RowaPhos, to reduce phosphate.
Normal to high levels of Ca and alkalinity also promote the growth of coralline algae, which will compete for nutrients with the slime, and cover available surfaces to prevent the spread of other nuisance algae. But it won't put much of a dent in the algae problem if nutrient (NO3 and PO4) levels are really high.
I have not kept seahorses or pipefish, so it's hard to know what is compatible with their needs. If you haven't, maybe pop over to
seahorse.org to see if there is something useful in their archives.
