How could I remove the central red line
Just apply:

[B,A]=butter(3,2*f0/fs,'high');
y=filter(B,A,x);


where fs is your sampling frequency, f0=5 (your red band-width, as shown approximatedly in your spectrogram), x is your nx1 column vector data and y is your new filtered data. 

A | B is the denominator | numerator of your highpass filter.

Hence the original picture: 

spectrogram(y);


will show the 0-5Hz band in a pure blue band, a sign of deleting everything in there...

Best regards....