Looks to me like the start of some red beard sponge. Clathria prolifera. If so, it's fairly common. It could also be a type of algae. Can you get a clearer picture? Here's an example of red beard sponge. It can vary in color from red to orange.
I personally like sponges in my tank and have several varieties. Some hate them. I guess it's a matter of personal preference. Some can spread rather quickly and would need to be kept in check. Some can be toxic as well so that's something to consider. If it is prolifera then the species name is a clue to its growth rate. A clearer pic. would still be best. I can't rule out algae.