I have just put up a new 5 gal. salt water tank. There is a weard red's alge growing. Is this part of the the aclimation of the tank with a alge bloom? Do i need a skimmer on such a small tank?

welcome BM4.

welcome to the salty ways & welcome to TR
If you've just set the tank up then yes, expect to see alot worst with yuk stuff growing, from green to red you should see it all. Its all types of bactera clutching on the life with the availible nutrients available in an new system, clean it over the next few weeks if it gets to much for you, but dont let it drive you nuts. As for the skimmer, theres no real rule in regards to skimmers. Truth be known skimmers are meant for larger water applications due to pollutant bonding contact time & the fact that 5gal in your case isn't awhole lot of water. So frequent water changes are a more realistic approach. personally Id rather spend money on an auto top off unit than a skimmer for nano type tanks. makes more sence from a stability standpoint, where as a skimmers effecency will be hindered or the oppersite, over skim

ok, thanks but it is not on the glass. It is only on the live rock and live sand.