I would guess the floating stuff is likely some type of fungus, or as you say bacteria, because they can give off a bad smell as they grow. I don't know what type of nutrients you are using but they don't all have the same color, and it would depend on how strong it is weather you would notice any color to it. At this point I wouldn't worry about any pH change, 5.5 to 6.5 should be fine for tomatoes, you are right in the middle of that.

precipitate generally sinks.
bacteria and fungus are more likely to float.

Thanks homehydro and grizzman for your comments.
I´m mixing potassium hydroxide with phosphoric acid, (0.3 g of KOH + 0.6 of H3PO4 (85%)) because in my country is difficult to find monopotassium phosphate. The other salts are the common nutrients: epsom salt, potassium and calcium nitrate, etc.