From a quick look it appears that it is more often pronounced as a J sound in the US (Orather than a hard G sound).
The singular of algae is alga. I would be surprised if that was still pronounced with a J sound, but my latin pronounciation is not perfect.
If I recall, latin always has 'hard' sounds for g and c (so no 'jiraffe' or 'see-sar'). Although if you were totally going by latin pronounciation you'd say 'al-guy'- the 'ae' ending for plural is pronounced 'aye' as in, puellae (girls) which is pronounced 'pu-ell-aye' as opposed to 'pu-ell-ee', which it would be if you pronounced it like most people are pronouncing algae.