The concrete flooring behind several Housing Board blocks in Woodlands was found to be covered in algae shortly after it had been repaved, prompting residents to alert the town council.
A photograph shared on forum discussion thread Reddit on Sunday shows dark green patches next to the void deck of several blocks in Woodlands Street 81.
The Reddit user said it has been "months" since the algae appeared.
It is not clear when the photo was taken.
Several other Reddit users said the algae growth could be a potential hazard, especially for the elderly folk living in the area, who could slip and injure themselves.
In response to queries, the Sembawang Town Council said the affected area is near Blocks 870 to 882, and around Blocks 801, 802 and 804 near Republic Polytechnic.
The town council spokesman said the algae growth was caused by poor workmanship from the re-rendering of the concrete surface, carried out between October last year and April. "We have discussed with the contractor in charge of the re-rendering, and it has agreed to carry out rectification works," added the spokesman.
Re-rendering refers to hacking off the concrete surface and reapplying fresh concrete over it.
Such concrete surfaces face natural deterioration, and will sustain wear and tear over time.
The spokesman said that it has asked the contractor to increase the frequency of the surface washing from once a month to once every fortnight in the meantime.
Chemicals will also be used to slow down algae growth while rectification works are being carried out.