From the Super Mario Wiki
Li'l Springies are extremely rare, indestructible enemies appearing in Super Mario Galaxy. They resemble yellowish brown elipses with two beady eyes and four sprouting springs. Li'l Springies move side to side in order to bounce and delay Mario, hindering the passage. Only four Li'l Springies appear in the entire game, only in Boo's Boneyard Galaxy. The player (or Boo Mario) cannot affect them at all.