Don't be alarmed. It is allowed. Below is the extract from
http://www.eld.gov.sg/pdf/PEC_handbook.pdf. However, next time maybe can pretend to approach and talk to them. They are not suppose to talk to voters.
6.6. Things that candidates/polling agents are allowed to do/prohibited from doing
Bear in mind that voting is compulsory. As such, it is an offence to dissuade someone from giving his vote at an election.
Candidates and polling agents must wear their official identification tags at all times while in any polling station. However, only the candidate will be allowed to wear party logos when visiting polling stations in the constituency he/she seeks election.