Dear Moon Valley:
As Leslie explained, algae often grows on the exoskeleton of seahorses, typically on their head and neck and upper back, which are closest to the light source. That’s perfectly normal and nothing at all to be concerned about. In fact, seahorses often encourage algae to grow on them as a protective device to enhance their camouflage, and it’s often best simply to ignore any such growth.
If you find the algae growth to be unsightly, you can certainly brush it off as Leslie described. Just be very careful when you are handling the seahorse and gently brushing away the algae so that you don’t remove any of the seahorse’s protective slime coat if you can possibly avoid it.
The seahorse’s pliant skin or integument is of course its first line of defense against disease. It contains mucus glands, and the slime covering the skin acts as a barrier to ectoparasites and infection. The protective slime can contain antibodies and antibacterial substances, and excessive mucus production is often the first sign of an infection or parasite problem. On the other hand, healthy seahorses often have small bright dots on their heads and torsos, which are actually mucus deposits. These beads of mucus glisten like little diamonds and are a sign of vibrant good health. When handling a seahorse, it’s important to wet your hands first in order to avoid removing too much of the protective slime coat.
Marine fish are always in danger of dehydration because the seawater they live in is saltier than their blood and internal body fluids. As a result, they are constantly losing water by diffusion (osmosis) through their gills and the surface of their skin, as well as in their urine. The mucus layer acts as a barrier against this, waterproofing the skin and reducing the amount of water that can diffuse across its surface.
So feel free to very gently brush away the algae as long as you take the necessary precautions and don’t disturb your seahorse’s protective slime coat any more than necessary, but you should be aware that the algae is very likely to quickly regrow if you don’t also address the conditions that are promoting the algae growth. Excess nutrients (especially nitrates and phosphates) fuel the growth of nuisance algae, so if you want to get rid of the algae growth on your seahorse once and for all, you should concentrate on reducing the nutrient loading in your aquarium and perhaps cut back on the photoperiod so the algae has less light for photosynthesis.
If you search this forum for the phrases "hair algae" or "slime algae" or "nuisance algae," you will find lots of detailed information and suggestions explaining how to get your nitrates and excess nutrients under control, which should be very helpful in preventing the recurrence of the algae growth on your seahorses.
Best wishes with all of your fishes, Moon Valley!