That isn't something that happened over night. That is a ton of many different algaes that is indicating there are some high nitrate and phosphate levels in the tank.
How often are water changes? Are you using ro/di water? How much and often are you feeding?
When it comes to not being able to add new fish, if the nitrates are high if they aren't being properly acclimated to the tank it can very easily shock and kill fish and if very high can be toxic.
WoW I got to agree with hank , looking at all that algae your parameters are most likely off the charts , so far off you will most likely get false readings
with nitrates and phosphates ,
An ideal suggestion is to put all the existing fish in a QT tank until you get this resolved I will explain further down this post, as for all that algae I see no good algae s , except 1
I see green hair now that's one of the biggest issues , than I see the invasive capulara it can over take a tank fast , those are the biggest bubble algae I've ever seen what ever you do don't pop them in the tank they'll spread like wild fire,
I do see a small patch of green grape macro to the bottom left of the pict that's the only beneficial algae I see , I saw that being a macro junkie
big water changes are in the tanks future , the capulara will need to be removed manually by hand ,
the bubble algae should be removed from the rock outside the tank manually wear glasses as they tend to squirt , than rinse the rock
the green hair can be tackled 1 of 2 ways 1 major water changes which could take around a month or so , or removal of the effected rock and do a 50/50 bath saltwater and peroxide using a stiff scrub brush than rinse any rocks with coral do the same but don't emerge corals in the bath , there will still be some hair there but it will die off fast the rocks can go back into tank,
Ok now this is why I suggested to remove fish ,
after rocks are reintroduced there may be a mini cycle , this could take around a week or 2 to settle since there will be a good amount of die off on the rocks
if your parents like the plant like appearance they should look into beneficial macro's and research them , macro's will help benefit the tank and fish health as some fish enjoy eating macro's , proper husbandry on a weekly basis that consists of water changes and some bi weekly trimming of the macros
brings a happy you and a healthy tank , and always do testing of your parameters weekly
early detections prevent big issues
research of both beneficial algaes and also nuisance algae will give you a better understanding of what each algae is both good or bad
this is a tank with beneficial macros
