See if this mini-key fits (from Huisman/Abbott/Smith 2007):
Plants not calcified.....1
1. Plants firm, irregularly globose or solid, composed of discrete cells.......2
1. Plant not as above, spongy....3
2. Plants encrusting or upright, cells abutting each other, not in a matrix.....DICTYOSPHAERIA
3. Plants either creeping or upright, epilithic, with terete branches or spongiose, composed of densely entwined filaments with an outer layer or elongate utricles (swollen ends of filaments)......CODIUM
If you say the plant is not calcified and is not spongy, I'd lean toward Dictyosphaeria. If it is calcified, then.....?? D. cavernosa made an appearance in my tank and is a common 'bubble' algae. The tight clusters of small vesicals is obvious in D. cavernosa and I can't make them out in your picture.