That last pic really looks like dinoflagellate to me and looks like you have a snail dying next to that bloom, knowing your water parameters would help it little more specially nitrate levels in phosphates.also how old is the tank?
if your water test ok you may want to take a water sample from about 1/2 inch into the sandbed and retest for phosphates.
I had that same situation and try to remove it for about three months I finally had to remove my sandbed.