Some water parameters would be nice,a few causes for green slime algae are,high phosphates,high nitrates,low ph,low alkalinity,insufficient water movement or current,high organics...If you have any of those problems,fix them and you won`t have to rely on a mechanical filter on the suction side of your pump,Every manufacturer of pumps makes a prefilter for their pumps....In the first section of your sump you mentioned skimming,Do you have a mechanical filter"Is that your inline" to catch larger debree in that area..If you are afraid of possibly starving your critters that you want to grow in your refugium just use a different micron rating on your sock,You can get filter socks that will filter out larger debree and allow smaller size food to suite the organisms you want to culture,all of the particles that settle in your sump.break down and add fuel to the fire so to speak.....With a 3-4 inch sand bed you are deffintly going to want some current running through your sump...... Just my two cents....