The proper term would be "these things", as they are animals, like corals, and as far as I know, generally they are not harmful. There are a few species that in fact bore into stony corals, but I do not know of anyone who has encountered them. More typically they have specific feeding/environmental requirements and die in aquaria. Some species do thrive, obviously, but sponges are not known as a choice for beginners.
An interesting quote from Bob Fenner: "...if you find the Orange Icing Sponge, Mycale laevis growing under your plate-type corals, this is not a 'bad thing'. This Sponge actually protects the Stony Coral from Boring Sponge infiltration."
http://www.wetwebmedia.com/sponges.htm
Chin_lee, does that thing have any visible polyps? I wonder if it could be some sort of soft coral?