You said you tested the water and it is fine, but what did you test for and what were the readings ?
I see a hang-on filter on the sump. Whats in it ? If it is carbon, when was the last time you changed it ? I would replace it with a large amount.
Take the Bio-max out of the sump.
Where did you get the sand from that you placed into the refuge part of the sump ? If you took it from inside of your tank and put it in there, you may have stirred up some nasty stuff and could be the cause of the death of your fish and the green water. Did you re-arrange the rock in your tank or add any new rock ? That could cause problems if detritus was trapped in /under them and now is in the water column.
A water change may be in order. If your nitrates are high then you may want to do a partial water change.
Shorten your photo-period. If that is an algae bloom, it feeds on light and nutrients.
I need more info to help you but that is what I can see from your pics.
You need a deep sand bed in your sump live rock micro algae plant. Or a algae Scrubber
Go on line and check it out a good refugium helps with less water changes. Also filter sock to help catch waste protein skimmer can't do it alone. And can put some carbon in filter bag to help take away smell and purify water. Read and research I'm new to this hobby so I read read, Good luck hope sump clears up.
Ol guys this is free form floating algae. Very common in fresh water tanks. Easiest way to clean this up is by a uv filter. I heard allgone also works but this is a bad case. O sure your phosphates are off the chats so u need to get those down. Don't tell me that ur test kit says ur pros levels are 0 because this has already been beat to death. Do some research on the topic but most off all practice better fish husbandry when u get this cleared up. If you are on top of that then change the filters in your ro/di and do some water changes every few days till the water clears up blow off you rocks with a turkey blaster or power head and churn up that sand to get any craps out of it.