I agree, the tank is a way too small for long term best chances of success.
The most likely source of the particles IMO, may be that you are not removing uneaten food before the next feeding, and/or, seahorses masticate their food when they snick it up, passing particulate matter out through the gills.
This though does look too large for it to be the particulate matter so I'd bet more on pieces of food that were not eaten.
I have found some packets of food contain a LOT more pieces rather than whole mysis shrimp, and most seahorses I've had tend to ignore these, as well as even whole mysis if it didn't look good to them.
If the pieces are food, then eventually you are going to have problems with bacteria infestations that will damage or kill your seahorses, especially with such a small tank.
I'd suggest in the short term at least, to find a way to filter that crap out of the tank, and then clean the filter daily or every other day so as not to create a "bedding" for the nasty bacteria.
Each of my tanks has a Hagen 802 power head with quick filter attachment, but I think now it's renamed Aquaclear 70. Anything though that will remove the particles is going to be a help.
http://ca-en.hagen.com/Aquatic/Pumps/Circulation/A570
http://ca-en.hagen.com/Aquatic/Pumps/Circulation/A575
Protein skimmers are a big aid to keeping seahorse tanks, but it's recommended that you buy one rated a WAY over the tank size you intend to use it for. Some have said even 5X tank size but I would think at the minimum, 2 or 3X would be needed.
Until you get them in a larger tank, you will have to do larger, more frequent water changes, and keep on top of any uneaten food that can be trapped out of sight, causing bacteria beds to develop as time goes on.
There are no hobby test kits that can test for this problem so it creeps up on us unknowingly, so take advantage of others bad experiences to try to not repeat them yourself.
I should also mention that a lot of us don't even use heaters in the tanks as the recommended temperature is 68° to 74°F and most homes are kept in that range.
Reasons for this are again, nasty bacteria the multiply exponentially above 74°F.
There are some excellent articles at the BOTTOM of "
My Thoughts on Seahorse Keeping" written by experienced keepers AND perhaps the best know commercial breeder of seahorses.