the fact that your tests are showing nitrites either means your tank hasn't fully cycled or your seeing the tail end of an ammonia spike which certainly wouldn't help your algae problem which is quite dangerous to your fish i would monitor that for the time being personally and maybe reduce feeding if it comes up again.. the best way to reduce your phosphate is with phosphate remover bagged up like your carbon in your external filter.
your magnesium and calcium is alittle low. also do you have a KH reading? your mushroom poylp should be fine as they are very hardy. my advice would be to monitor it closely for now to see if it deteriorates or improves. is it a variety that has zooxanthellate or not? if it doesn't then you can see if it excepts some meaty foods maybe? i have two mushroom polyps that are very hungry. ive watched the largest one eat a whole prawn before!!
sounds like diatom algae. can you post up your readings for ammonia, nitrite, nitrate etc? and if you have a phosphate reading that to? it can be a symptom of a new tank and normally burns its self out.
it was snowing here in braintree earlyer. might as well be snowing inside too though as my heating packed up yesterday. the boiler men came today and "fixed it" ie fobbed my other half off with a load of nonsense so another cold bath for me then!! joy.