Is it ok if lead acid battery charge with too low current?
A low current trickle charge is better than no charge at all.  

If you can keep the cells topped up with distilled water you should not cause any real damage even with a charge that is somewhat larger than optimal.  

However at 2.5V per cell you are on the low side for real charging (spiral wound 'Cyclon' cells excluded where it is more acceptable) and should try to get a bit more voltage at about 2.8V per cell or 5.6V for your two cell battery.  

Most of the current in the charging direction will be used for charging, less any internal discharge current due to age, electrolyte contamination or other damage.  It will charge very slowly (88 hours with constant current and 100% efficiency However the current should fall close to zero when the cell voltage has reached 2.5V) and never reach a full charge though.

Your charger may be able to supply slightly more voltage to reach a full charge very slowly if it does not have tight regulation but this is not a sure thing.