ALT CRZ implies CRZ phase, FlapsOne is correct, and in practice that means no change from current speed unless you are flying slower than EO CRZ SPD. Not Green Dot 'cos the other engine(s) would reduce thrust, which is normally undesirable at this point.
ALT implies CLB or DES phase: target speed is set to EO CRZ SPD at the FCU altitude (i.e. current speed is not an option). Always subject to limit speed.
Cheers, TP