If you just don't clean the water tank or keep a very shallow pond, all the algae they would ever want will grow free of charge and you don't even have to harvest it. It will even self medicate the animal as they can choose when and how much of the algae they ingest. The good slimy algae probably works best especially if they can access some lilly pads to add a little flavor once in a while.
The best source of food grade kelp can be found in the "Total Equine " horse feed. Since it is included in the formula, it is assimilated in the proper amount for the best results fo any horse. By adjusting the amount you feed you have complete control to adjust for the tpe of horse you have. Many people I know that have problem horses or "Hard Keepers" have had incedible success with this feed.
I do feed the Total Equine. So far results are good. Fat horse lost some fat thin horse gained muscle. I am hoping it helps this summer with the heat. The fat horse has problems in the heat & humidity. This is my last ditch effort before I sell her.
Kelp(other seaplants) has a wide array of trace minerals. It also has a very high iodine content, so if the horse's diet is iodine deficient, as they often are, I think it's a great supplement. But if the horse already gets adequate iodine, I wouldn't feed it, or ensure it's not fed at levels that will OD the horse on iodine.