Google Earth Engine: Error for mapping expression function over feature collection

I’m having an issue when trying to create a function to map over a feature collection.

I’m trying to create SST thresholds for several regions where over a certain SST value is 1, under another SST, value is 0 and in between is a linear function between those cut offs.

I tried this with a single image first and it looked like it had worked. Link here

However, I’d like to apply this to a featurecollection I’ve created , so I wanted to create a function of this and then map it over the collection. However I’m getting an error

Invalid argument specified for ee.List(): mean.

I think it is because I’m not defining mean correctly in the expression. I’ve tried lots of different options and can’t figure it out. The code I’m using is here

var sst = ee.ImageCollection('NASA/OCEANDATA/MODIS-Aqua/L3SMI').select('sst')
.filterDate(startDate, endDate)
.filterBounds(reefs_buffer)
.map(function(image){return image.clip(EEZ)}); //reefs_buffer

var sstMean = sst.reduce(ee.Reducer.mean())

print('sstMean', sstMean)

var reefs_sstMean = sstMean.reduceRegions({
  reducer: ee.Reducer.mean(),
  collection: reefs_buffer,
  scale: 30});

 print('reefs_sstMean', reefs_sstMean)

function sstHS_function(feature) {
   var sstReefs = feature.select('mean');
   var sstHS = sstReefs.expression(
"(b('mean') > 29.5) ? 1" +
 ": (b('mean')  < 24) ? 0" +
 ": 2/11*mean - 48/11 ",
 {'mean' : mean});
   return ee.Feature(sstHS);
 }

 var sstHS_collection = reefs_sstMean.map(sstHS_function)
 print(sstHS_collection)
 reefs_sstMean.map(sstHS_function)