Your alage problem is something most of us have fought/are fighting and there are a couple things that'll help rid your tank of it - but it'll take TIME - like weeks & months. Alage esp GHA needs light/nitrates/phosphates to flourish. Reducing photoperiod will help - I've run as little as 4 hours/day. ALSO old bulbs - either MH/HQI OR flourescent will really help alage grow. As bulbs age/run - they begin "red-shifting" or producing more red&orange spectrum light....GHA loves red & orange light - so halide bulbs over 12 months old or florescents older than 6-8 months would be suspect and might should be changed.
primarily, your nitrates&phosphates are high, and that's feeding the gha. Even though your tests may not show very high readings is likely due to 2 things, first the GHA is absorbing/growing on the excess nutrients thus making NO3 & PO4 levels "look" low - cause the GHA uses it up and 2nd, few tests are really very accurate - esp at very LOW levels - which is what your tank MAY show - but if you took away all the gha, your no3/po4 numbers would likely skyrocket - cause ther'd be nothing to consume it. Also test your water-source for problems too. My rodi filter needs at least 2 of 5 stages changed about 2x per year to maintain 000tds. making "new" saltwater that already has 'trates & 'phates in it to start with is pointless. Also reducing feeding and reducing bio-load (# of fish) may be necessary. If your 90g is heavily stocked with fish - you might need to consider adding a GFO reactor to assist with high phates.. Maybe a turf scrubber - though research that well
sadly, it's really hard to rid a tank of chronic GHA - cause your gonna have to starve the gha of light & nutrients for weeks/months while it dies off and there's no 1 thing or easy fix that'll rid it. It will take a combo of all the things mentioned